Block copolymer composition for flexographic plate

ABSTRACT

The present disclosure provides a block copolymer composition for a flexographic plate comprising a block copolymer A represented by the following general formula (A) and a block copolymer B represented by the following general formula (B), wherein: a content of aromatic vinyl monomer units relative to all monomer units constituting polymer components of the block copolymer composition for the flexographic plate is 18% to 70% by mass; a type A hardness is 25 to 65; and an anisotropic index is 2.0 or less.

TECHNICAL FIELD

The present invention relates to a block copolymer composition for aflexographic plate comprising a block copolymer composition composed ofa block copolymer including a conjugated diene polymer block and anaromatic vinyl polymer block. In more specific, the present inventionrelates to a block copolymer composition for a flexographic plate havingexcellent flexibility and small anisotropy, with which a flexographicplate having high definition printing properties and excellentink-swelling resistance and abrasion resistance can be provided.

BACKGROUND ART

Flexographic printing has been broadly used as a printing method formaterials such as a label, a plastic container, a carton, a plastic bag,a box, and an envelope. As a flexographic plate used for thisflexographic printing, the one frequently used is formed by exposing aphotosensitive composition for a flexographic plate comprising anelastomer, a polymerizable ethyleny unsaturated monomer, and aphotopolymerization initiator.

The photosensitive composition for a flexographic plate is usuallyformed in a sheet shape, supplied as a sheet having a multi-layeredstructure wherein a flexible sheet as a supporting body is arranged onone surface and a protective film is arranged on the other surface.Light is irradiated to this multi-layered sheet from the supporting bodyside to cure a specified thickness of the photosensitive compositionlayer for a flexographic plate. Next, the protective film is peeled offand a negative film is adhered to that surface, and then light isirradiated from above the said negative film. The part of thephotosensitive composition for a flexographic plate where the light hasbeen transmitted is cured, and the part not cured are removed bysubstance such as an organic solvent and an aqueous solvent. In thismanner, a flexographic plate having a concave and convex structure isformed.

As an elastomer used to constitute the photosensitive composition for aflexographic plate, a variety of thermoplastic elastomers havingexcellent workability have been broadly used. Among them, an aromaticvinyl-conjugated diene-aromatic vinyl block copolymer such as astyrene-isoprene-styrene block copolymer (SIS) and astyrene-butadiene-styrene block copolymer (SBS) has been usedapprovingly as an elastomer for constituting the photosensitivecomposition for a flexographic plate, since the said copolymer has highrubber elasticity, and flexibility and impact resilience suitable forconstituting a flexographic plate. In addition, properties such asabrasion resistance and ink-swelling resistance are also required forthe photosensitive composition for a flexographic plate in order toallow high definition printing. For that reason, various researches havebeen conducted to improve the aromatic vinyl-conjugated diene-aromaticvinyl block copolymer used for constituting the photosensitivecomposition for a flexographic plate.

For example, Patent Literature 1 discloses: a photosensitive blockcopolymer for a printing plate material comprising an aromaticvinyl-conjugated diene block copolymer, wherein an aromatic vinylcompound content is 10 to 40 mass %, a toluene insoluble is 30 ppm orless, a type A hardness is 85 or less, and a vinyl bond content in aconjugated diene polymer block is 50% or less; and a photosensitiveblock copolymer composition for a printing plate material and aphotosensitive elastomer composition including the above describedphotosensitive block copolymer for a printing plate material. It isdescribed that the said photosensitive block copolymer for a printingplate material has a small amount of gel and excellent in processstability, image developing properties and printing properties.

In addition, Patent Literature 2 discloses a photopolymerizablecomposition for a flexographic plate including a mixture of SIS and SBS.It is described that this photopolymerizable composition has highabrasion resistance, and is excellent in flexibility not havingexcessive hardness. It is also described that the saidphotopolymerizable composition does not express anisotropy caused bymolecular orientation which occurs during a melt process; thus, negativeeffects to printing that occur when a flexographic plate formed by amaterial including anisotropy is used may be avoided.

In addition, Patent Literature 3 discloses a photopolymerizablecomposition for a flexographic plate including an aromaticvinyl-conjugated diene-aromatic vinyl block copolymer wherein aconjugated diene polymer block is a random copolymer block of isopreneand butadiene. It is described that the said photopolymerizablecomposition is excellent in transparency.

Further, Patent Literature 4 discloses a block copolymer composition fora flexographic plate including a three-branched-type aromaticvinyl-conjugated diene block copolymer obtained by using a specifiedcoupling agent. The said block copolymer composition for a flexographicplate when constituting a flexographic plate is excellent in smoothnessand flow resistance (properties not easily deformed before curing bybeing exposed to light), and reproductivity of fine lines.

However, it has been difficult to obtain a composition havingwell-balanced properties such as abrasion resistance and flexibility,even when the compositions described in Patent Literatures 1 to 4 areused.

In other words, it has been known that the properties such as abrasionresistance of the aromatic vinyl-conjugated diene-aromatic vinyl blockcopolymer may be improved by increasing the proportion of an aromaticvinyl monomer unit included therein to increase mechanical strength.However, there has been a problem that the polymer loses rubberelasticity when the proportion of the aromatic vinyl monomer unit isincreased to a level with which sufficient abrasion resistance for useas a flexographic plate is obtained.

CITATION LIST Patent Literatures

Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No.2006-104359

Patent Literature 2: JP-A No. 2002-072457

Patent Literature 3: Japanese Unexamined Patent Publication No.2006-514338

Patent Literature 4: WO2005/031459

SUMMARY OF INVENTION Technical Problem

The object of the present invention is to provide a block copolymercomposition for a flexographic plate having excellent flexibility andsmall anisotropy, that can provide a flexographic plate having highdefinition printing properties and excellent ink-swelling resistance andabrasion resistance.

Solution to Problem

The inventors of the present invention conducted a thoroughinvestigation in order to achieve the object described above, and as aresult, they found that, with a block copolymer composition includingtwo kinds of block copolymers comprising a conjugated diene polymerblock and an aromatic vinyl polymer block respectively having aspecified structure, wherein a type A hardness and an anisotropic indexare in specified ranges, the balance of abrasion resistance and rubberelasticity is excellent even when the proportion of the aromatic vinylmonomer unit is increased. Moreover, the block copolymer composition hasexcellent crosslinking properties and small anisotropy; thus, aflexographic plate having high definition printing properties andexcellent ink-swelling resistance and abrasion resistance may beobtained using the said block copolymer composition. The presentinvention has been completed based on these findings.

Thus, according to the present invention, there is provided a blockcopolymer composition for a flexographic plate, comprising a blockcopolymer A represented by the following general formula (A) and a blockcopolymer B represented by the following general formula (B), wherein: acontent of aromatic vinyl monomer units relative to all monomer unitsconstituting polymer components of the block copolymer composition forthe flexographic plate is 18% to 70% by mass; a type A hardness is 25 to65; and an anisotropic index is 2.0 or less.

Ar1^(a)-D^(a)-Ar2^(a)  (A)

(Ar^(b)-D^(b))_(n)-X  (B)

(In the general formula (A) and the general formula (B), Ar1^(a) andAr^(b) each represent an aromatic vinyl polymer block having a weightaverage molecular weight of 6,000 to 20,000; Ar2^(a) represents anaromatic vinyl polymer block having a weight average molecular weight of25,000 to 400,000, a ratio (Mw(Ar2^(a))/Mw(Ar1^(a))) of the weightaverage molecular weight of Ar2^(a) and the weight average molecularweight of Ar1^(a) is 2 to 20; D^(a) and D^(b) each represent aconjugated diene polymer block having a vinyl bond content of 21% to 70%by mol; X represents a single bond or a residue of coupling agent; and nrepresents an integer of 2 or more.)

It is preferable that, in the block copolymer composition for aflexographic plate, the content of aromatic vinyl monomer units relativeto all monomer units constituting polymer components of the blockcopolymer composition for the flexographic plate is 20% to 70% by mass.

It is preferable that, in the block copolymer composition for aflexographic plate, a mass ratio (A/B) of the block copolymer A and theblock copolymer B is 36/64 to 85/15.

It is preferable that, the block copolymer composition for aflexographic plate further comprises a block copolymer C represented bythe following general formula (C).

Ar^(c)-D^(c)  (C)

(In the general formula (C), Ar^(c) represents an aromatic vinyl polymerblock having a weight average molecular weight of 6,000 to 20,000, andD^(c) represents a conjugated diene polymer block having a vinyl bondcontent of 21% to 70% by mol.)

It is preferable that, in the block copolymer composition for aflexographic plate, the block copolymer B is obtained by using acompound including two or more of at least one kind of functional groupselected from an alkoxyl group, an ester group and an epoxy group in onemolecule as a coupling agent.

Also, according to the present invention, there is provided aphotosensitive composition for a flexographic plate comprising the abovedescribed block copolymer composition for a flexographic plate, anethyleny unsaturated compound having a molecular weight of 5,000 orless, and a photopolymerization initiator.

Further, according to the present invention, there is provided aflexographic plate comprising the above described photosensitivecomposition for a flexographic plate.

In addition, according to the present invention, there is provided amethod for producing the above described block copolymer composition fora flexographic plate, the method comprising: a first step of forming anaromatic vinyl polymer including an active terminal by polymerizing anaromatic vinyl monomer using a polymerization initiator in a solvent; asecond step of forming an aromatic vinyl-conjugated diene blockcopolymer including an active terminal by adding a conjugated dienemonomer to a solution obtained in the first step for polymerization; athird step of forming a block copolymer B by adding, as a couplingagent, a compound including two or more of at least one kind offunctional group selected from an alkoxyl group, an ester group and anepoxy group in one molecule, to a solution obtained in the second step,in an amount the functional group relative to the active terminal isless than one equimolar amount; a fourth step of forming a blockcopolymer A by adding an aromatic vinyl monomer to a solution obtainedin the third step for polymerization; and a fifth step of collecting theblock copolymer composition for the flexographic plate from a solutionobtained in the fourth step; wherein reactions in at least the secondstep, the third step, and the fourth step are conducted under thepresence of a Lewis base compound, and an amount of the Lewis basecompound per 1 mol of the active terminal in the polymerizationinitiator is 0.1 to 50 mol.

Advantageous Effects of Invention

According to the present invention, a block copolymer composition for aflexographic plate having excellent flexibility and small anisotropythat can provide a flexographic plate having high definition printingproperties and excellent ink-swelling resistance and abrasion resistancemay be obtained.

DESCRIPTION OF EMBODIMENTS

The block copolymer composition for a flexographic plate and theproduction method therefor, the photosensitive composition for aflexographic plate as well as the flexographic plate of the presentinvention will be hereinafter described.

A. Block Copolymer Composition for Flexographic Plate

The block copolymer composition for a flexographic plate of the presentinvention comprises at least two kinds of block copolymers. One of thetwo kinds of bloc copolymers constituting the block copolymer for aflexographic plate of the present invention, the block copolymer A is anaromatic vinyl-conjugated diene-aromatic vinyl block copolymer,represented by the following general formula (A), including two ofaromatic vinyl polymer blocks respectively having different weightaverage molecular weight from each other.

Ar1^(a)-D^(a)-Ar2^(a)  (A)

In the above general formula (A), Ar1^(a) represents an aromatic vinylpolymer block having a weight average molecular weight of 6,000 to20,000; Ar2^(a) represents an aromatic vinyl polymer block having aweight average molecular weight of 25,000 to 400,000, a ratio(Mw(Ar2^(a))/Mw(Ar1^(a))) of the weight average molecular weight ofAr2^(a) and the weight average molecular weight of Ar1^(a) is 2 to 20;and D^(a) represents a conjugated diene polymer block having a vinylbond content of 21% to 70% by mol.

Also, the other of the two kinds of bloc copolymers constituting theblock copolymer for a flexographic plate of the present invention, theblock copolymer B is an aromatic vinyl-conjugated diene-aromatic vinylblock copolymer represented by the following general formula (B).

(Ar^(b)-D^(b))_(n)-X  (B)

In the above general formula (B), Ar^(b) represents an aromatic vinylpolymer block having a weight average molecular weight of 6,000 to20,000; D^(b) represents a conjugated diene polymer block having a vinylbond content of 21% to 70% by mol; X represents a single bond or aresidue of coupling agent; and n represents an integer of 2 or more.

The block copolymer composition for a flexographic plate of the presentinvention may further comprise a block copolymer C, that is an aromaticvinyl-conjugated diene block copolymer represented by the followinggeneral formula (C), in addition to the block copolymer A and the blockcopolymer B.

Ar^(c)-D^(c)  (C)

In the above general formula (C), Ar^(c) represents an aromatic vinylpolymer block having a weight average molecular weight of 6,000 to20,000, and D^(c) represents a conjugated diene polymer block having avinyl bond content of 21% to 70% by mol.

The aromatic vinyl polymer blocks (Ar1^(a), Ar2^(a), Ar^(b), and Ar^(c))in the block copolymers A to C are respectively a polymer block formedfrom an aromatic vinyl monomer unit. There are no particular limitationson the aromatic vinyl monomer used to constitute the aromatic vinylmonomer unit of the aromatic vinyl polymer blocks, and examples thereofmay include styrene, α-methylstyrene, 2-methylstyrene, 3-methylstyrene,4-methylstyrene, 2-ethylstyrene, 3-ethylstyrene, 4-ethylstyrene,2,4-diisopropylstyrene, 2,4-dimethylstyrene, 4-t-butylstyrene,5-t-butyl-2-methylstyrene, 2-chlorostyrene, 3-chlorostyrene,4-chlorostyrene, 4-bromostyrene, 2-methyl-4,6-dichlorostyrene,2,4-dibromostyrene, and vinylnaphthalene. Among these, styrene ispreferably used. These aromatic vinyl monomers may be used each singlyor in combination of two or more kinds thereof, in each of the aromaticvinyl polymer blocks. Also, in each of the aromatic vinyl polymerblocks, same aromatic vinyl monomer may be used, and different aromaticvinyl monomer may respectively be used.

The aromatic vinyl polymer blocks (Ar1^(a), Ar2^(a), Ar^(b), and Ar^(c))in the block copolymers A to C may respectively include a monomer unitother than the aromatic vinyl monomer unit. Examples of the monomer thatconstitutes the monomer unit other than the aromatic vinyl monomer unitthat can be included in the aromatic vinyl polymer blocks may include,conjugated diene monomers such as 1,3-butadiene andisoprene(2-methyl-1,3-butadiene), α,β-unsaturated nitrile monomer,unsaturated carboxylic acid or acid anhydride monomer, unsaturatedcarboxylic ester monomer, and nonconjugated diene monomer. The contentof the monomer unit other than the aromatic vinyl monomer unit in eachof the aromatic vinyl polymer block is preferably 20% by mass or less,more preferably 10% by mass or less, and particularly preferablysubstantially 0% by mass.

The conjugated diene polymer blocks (D^(a), D^(b), and D^(c)) in theblock copolymers A to C are respectively a polymer block formed from aconjugated diene monomer unit. There are no particular limitations onthe conjugated diene used to constitute the conjugated diene monomerunit of the conjugated diene polymer block if it is a conjugated dienecompound, and examples thereof may include 1,3-butadiene, isoprene,2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, 1,3-pentadiene, and1,3-hexadiene. Among these, it is preferable to use 1,3-butadiene and/orisoprene, and it is particularly preferable to use isoprene. When theconjugated diene polymer block is formed from an isoprene unit, theblock copolymer composition for a flexographic plate to be obtained maybe highly flexible and may provide a flexographic plate excellent inflexibility. These conjugated diene monomers can be respectively usedsingly or in combination of two or more kinds thereof in each of theconjugated diene polymer blocks. Also, in each of the conjugated dienepolymer blocks, same conjugated diene monomer may be used, and differentconjugated diene monomer may respectively be used. Further, some of theunsaturated bonds in each of the conjugated diene polymer blocks may besubjected to a hydrogenation reaction.

The conjugated diene polymer blocks (D^(a), D^(b), and D^(c)) in theblock copolymers A to C may respectively include a monomer unit otherthan the conjugated diene monomer unit. Examples of the monomer thatconstitutes the monomer unit other than the conjugated diene monomerunit that can be included in the conjugated diene polymer block mayinclude, aromatic vinyl monomers such as styrene and α-methylstyrene,α,β-unsaturated nitrile monomers, unsaturated carboxylic acid or acidanhydride monomers, unsaturated carboxylic acid ester monomers, andnon-conjugated diene monomers. The content of the monomer unit otherthan the conjugated diene monomer unit in each of the conjugated dienepolymer blocks is preferably 20% by mass or less, more preferably 10% bymass or less, and particularly preferably substantially 0% by mass.

The block copolymer A that constitutes the block copolymer compositionfor a flexographic plate of the present invention is an asymmetricaromatic vinyl-conjugated diene-aromatic vinyl block copolymerrepresented in the above general formula (A), wherein the aromatic vinylpolymer block Ar1^(a) having relatively small weight average molecularweight, the conjugated diene polymer block D^(a) including the specifiedvinyl bond content, and the aromatic vinyl polymer block Ar2^(a) havingrelatively large weight average molecular weight are linked in thisorder.

The weight average molecular weight (Mw(Ar1^(a))) of the aromatic vinylpolymer block Ar1^(a) having relatively small weight average molecularweight is 6,000 to 20,000, preferably 7,000 to 18,000, and morepreferably 8,000 to 16,000. If the Mw(Ar1^(a)) is out of this range,there is a risk that the rubber elasticity of the block copolymercomposition for a flexographic plate to be obtained may be insufficient.

Also, the weight average molecular weight (Mw(Ar2^(a))) of the aromaticvinyl polymer block Ar2^(a) having relatively large weight averagemolecular weight is 25,000 to 400,000, preferably 25,000 to 300,000, andmore preferably 25,000 to 200,000. If the Mw(Ar2^(a)) is too small,there is a risk that the block copolymer composition for a flexographicplate to be obtained may be inferior in abrasion resistance and mayeasily express anisotropy. In addition, it may be difficult in somecases to produce the block copolymer A having too large Mw(Ar2^(a)).

Incidentally, in the present invention, the weight average molecularweight of the polymer or polymer block will be determined as a valuemeasured by high performance liquid chromatography and calculatedrelative to polystyrene standards.

In the block copolymer A, the ratio Mw(Ar2^(a))/Mw(Ar1^(a)) of theweight average molecular weight Mw(Ar2^(a)) of the aromatic vinylpolymer block Ar2^(a) having relatively large weight average molecularweight and the aromatic vinyl polymer block Mw(Ar1^(a)) of the aromaticvinyl polymer block Ar1^(a) having relatively small weight averagemolecular weight is 2 to 20, preferably 2 to 18, and more preferably 2to 16. The block copolymer A is formed in this way, and thus the blockcopolymer composition for a flexographic plate to be obtained maymaintain rubber elasticity even when the proportion of the aromaticvinyl monomer unit therein is increased to have excellent abrasionresistance.

The vinyl bond content of the conjugated diene polymer block D^(a) ofthe block copolymer A (the content of 1,2-vinyl bonds and 3,4-vinylbonds relative to all conjugated diene monomer units) is 21% to 70% bymole, preferably 21% to 65% by mole, and more preferably 21% to 60% bymole. The vinyl bond content in the above described range allows thecrosslinking properties of the block copolymer composition for aflexographic plate to be excellent, and thus clear images may be formedin a flexographic plate while the durability of the image may beimproved. Incidentally, it is considered that increasing the amount oflight irradiation during curing a photosensitive composition for aflexographic plate including the block copolymer composition for aflexographic plate may also improve the level of crosslinking. However,influences such as halation increase along with the increase in theamount of light irradiation to deteriorate resolution; as a result,there is a risk that the printing images may be unclear.

The weight average molecular weight Mw(D^(a)) of the conjugated dienepolymer block D^(a) of the block copolymer A is not particularlylimited, but is usually 40,000 to 200,000, preferably 42,000 to 180,000,and more preferably 4,5000 to 150,000. The Mw(D^(a)) is in the abovedescribed range, and thus the block copolymer composition for aflexographic plate to be obtained may maintain rubber elasticity evenwhen the proportion of the aromatic vinyl monomer unit therein isincreased to have excellent abrasion resistance.

The content of the aromatic vinyl monomer unit relative to all monomerunits in the block copolymer A is not particularly limited, but ispreferably 41% or more, more preferably 43% to 95% by mass, andparticularly preferably 45% to 90% by mass. The reason therefor is thatwhen the content of the aromatic vinyl monomer unit relative to allmonomer units in the block copolymer A is in the above described range,the block copolymer composition for a flexographic plate to be obtainedmay be excellent in abrasion resistance and ink-swelling resistance.

The weight average molecular weight of the block copolymer A as a wholeis not particularly limited, but is usually 70,000 to 500,000,preferably 80,000 to 450,000, and more preferably 90,000 to 400,000.

The block copolymer B that constitutes the block copolymer compositionfor a flexographic plate of the present invention is, as represented inthe general formula (B), a block copolymer formed by two or more of thediblock Ar^(b)-D^(b), which is formed by the aromatic vinyl polymerblock Ar^(b) having a specified weight average molecular weight bondedto the conjugated diene polymer block D^(b) having a specified vinylbond content, bonded directly in a single bond or bonded interposing theresidue of coupling agent.

The weight average molecular weight Mw(Ar^(b)) of the aromatic vinylpolymer block Ar^(b) that constitutes the block copolymer B is 6,000 to20,000, and is preferably 7,000 to 18,000, and more preferably 8,000 to16,000. If the Mw(Ar^(b)) is too small, there is a risk that the blockcopolymer composition for a flexographic plate to be obtained may haveinferior abrasion resistance, and if the Mw(Ar^(b)) is too large, thereis a risk that the composition may have inferior flexibility or rubberelasticity. The weight average molecular weight Mw(Ar^(b)) of thearomatic vinyl polymer block plurally included in the block copolymer Bmay be equal in each polymer block and may be different in each polymerblock as long as in the above described range, but is preferablysubstantially equal in each polymer block. In addition, each of theweight average molecular weight Mw(Ar^(b)) of these aromatic vinylpolymer blocks is more preferably substantially equal to the weightaverage molecular weight Mw(Ar1^(a)) of the aromatic vinyl polymer blockAr1^(a) having relatively small weight average molecular weight in theblock copolymer A.

The vinyl bond content of the conjugated diene polymer block D^(b) ofthe block copolymer B is 21% to 70% by mole, preferably 21% to 65% bymole, and more preferably 21% to 60% by mole. The vinyl bond content inthe above described range allows the crosslinking properties of theblock copolymer composition for a flexographic plate to be excellent,and thus clear images may be formed in a flexographic plate while thedurability of the image may be improved. In addition, the vinyl bondcontent of the conjugated diene polymer block D^(b) of the blockcopolymer B is preferably substantially equal to the vinyl bond contentof the conjugated diene polymer block D^(a) of the block copolymer A.

The block copolymer B is formed by the diblocks Ar^(b)-D^(b), which isformed by the aromatic vinyl polymer block Ar^(b) bonded to theconjugated diene polymer block D^(b), bonded directly in a single bondor bonded interposing the residue of coupling agent.

The coupling agent that constitutes the residue of coupling agent is notparticularly limited, and is an arbitrary coupling agent that isbifunctional or more. Examples of bifunctional coupling agent mayinclude bifunctional halogenated silane such as dichlorosilane,monomethyldichlorosilane, and dimethyldichlorosilane; bifunctionalalkoxysilane such as diphenyldimethoxysilane and diphenyldiethoxysilane;bifunctional halogenated alkane such as dichloroethane, dibromoethane,methylene chloride, and dibromomethane; bifunctional halogenated tinsuch as dichloro-tin, monomethyldichlorotin, dimethyldichlorotin,monoethyldichlorotin, diethyldichlorotin, monobutyldichlorotin, anddibutyldichlorotin; dibromobenzene, benzoic acid, carbon monoxide, and2-chloropropene. Examples of trifunctional coupling agent may includetrifunctional halogenated alkane such as trichloroethane andtrichloropropane; trifunctional halogenated silane such asmethyltrichlorosilane and ethyltrichlorosilane; and trifunctionalalkoxysilane such as methyltrimethoxysilane, phenyltrimethoxysilane, andphenyltriethoxysilane. Examples of tetrafunctional coupling agent mayinclude tetrafunctional halogenated alkane such as carbon tetrachloride,carbon tetrabromide, and tetrachloroethane; tetrafunctional halogenatedsilane such as tetrachlorosilane and tetrabromosilane; tetrafunctionalalkoxysilane such as tetramethoxysilane and tetraethoxysilane; andtetrafunctional halogenated tin such as tetrachlorotin andtetrabromotin. Examples of pentafunctional or more coupling agent mayinclude 1,1,1,2,2-pentachloroethane, perchloroethane,pentachlorobenzene, perchlorobenzene, octabromodiphenylether, anddecabromodiphenylether. These coupling agents may be used singly in onekind, or two kinds or more thereof may be used in combination.

In addition, in order to obtain the block copolymer B, among thesecoupling agents, it is preferable to use a compound including two ormore of at least one kind of the functional group selected from analkoxyl group, an ester group, and an epoxy group in one molecular as afunctional group that reacts with the active terminal of the polymer,and it is particularly preferable to use an alkoxy silane compoundincluding two or more alkoxy groups directly bonded to a silicon atomper one molecule. That is, the block copolymer B that constitutes theblock copolymer composition for a flexographic plate of the presentinvention is preferably obtained by using the compound including two ormore of at least one kind of the functional group selected from analkoxyl group, an ester group, and an epoxy group in one molecular as acoupling agent, and is particularly preferably obtained by using analkoxy silane compound including two or more alkoxy groups directlybonded to a silicon atom per one molecule, as a coupling agent. Use ofsuch a coupling agent allows the block copolymer composition for aflexographic plate to have excellent transparency, and it may be easy toobtain a flexographic plate in which fine printing patterns are formedfrom the obtained photosensitive composition for a flexographic plate.

In the block copolymer B, the number of the bond of the diblockAr^(b)-D^(b), that is n in the general formula (B), is not particularlylimited if it is 2 or more; the block copolymers B having differentnumber of the bond of the diblock may be mixed. The n in the generalformula (B) is not particularly limited if it is an integer of 2 ormore, but is usually an integer of 2 to 8, and preferably an integer of2 to 4. Also, as at least as a part of the block copolymer B, it isparticularly preferable that three or more of the diblock Ar^(b)-D^(b)are bonded interposing the coupling agent (that is, n is 3 or more inthe general formula (B)). The block copolymer composition for aflexographic plate of the present invention may provide a flexographicplate having high isotropy that has homogeneous mechanicalcharacteristics over all directions even when a forming method thateasily cause molecular orientation such as an extrusion molding methodis used when the flexographic plate is produced. When three or more ofthe diblock Ar^(b)-D^(b) bonded interposing the coupling agent isincluded as at least a part of the block copolymer B, a flexographicplate particularly high in isotropy and not easily causing printingdefect may be obtained.

The weight average molecular weight Mw(D^(b)) of the conjugated dienepolymer block D^(b) of the block copolymer B is not particularlylimited, but is usually 40,000 to 200,000, and is preferably 42,000 to180,000, and more preferably 45,000 to 150,000. The Mw(D^(b)) is in theabove described range, and thus the block copolymer composition for aflexographic plate to be obtained may maintain rubber elasticity evenwhen the proportion of the aromatic vinyl monomer unit therein isincreased to have excellent abrasion resistance. In addition, the weightaverage molecular weight Mw(D^(b)) of the conjugated diene polymer blockD^(b) of the block copolymer B is preferably substantially equal to theweight average molecular weight Mw(D^(a)) of the conjugated dienepolymer block D^(a) of the block copolymer A.

Incidentally, when an aromatic vinyl-conjugated diene-aromatic vinylblock copolymer produced without using a coupling agent is used as theblock copolymer B, the conjugated diene polymer block included thereinis formed by all the monomer units directly bonded, that issubstantially not formed from two of the conjugated diene polymer blockD^(b). However, in the present invention, such a conjugated dienepolymer block is also conceptionally treated as the polymer blockwherein the two of the conjugated diene polymer block D^(b) having asubstantially equal weight average molecular weight are bonded in asingle bond. Thus, for example, in the block copolymer B that is anaromatic vinyl-conjugated diene-aromatic vinyl block copolymer producedwithout using a coupling agent, when the conjugated diene polymer blockhas the weight average molecular weight of 100,000 as a whole, Mw(D^(b))thereof is treated as 50,000.

The content of the aromatic vinyl monomer unit relative to all monomerunits in the block copolymer B is not particularly limited, but isusually 10% to 35% by mass, preferably 11% to 32% by mass, and morepreferably 12% to 30% by mass.

The weight average molecular weight of the block copolymer B as a wholeis also not particularly limited, but is usually 52,000 to 800,000,preferably 70,000 to 600,000, and more preferably 100,000 to 400,000.

The block copolymer C that can be included in the block copolymercomposition for a flexographic plate of the present invention is, asrepresented by the above general formula (C), a block copolymer formedby the aromatic vinyl polymer block Ar^(c) having a specified weightaverage molecular weight bonded to the conjugated diene polymer blockD^(c) having a specified vinyl bond content. When this block copolymer Cis included, the block copolymer composition for a flexographic platemay have particularly well-balanced in abrasion resistance and rubberelasticity.

The weight average molecular weight Mw(Ar^(c)) of the aromatic vinylpolymer block Ar^(c) that constitutes the block copolymer C is 6,000 to20,000, preferably 7,000 to 18,000, and more preferably 8,000 to 16,000.Furthermore, it is preferable that the weight average molecular weightMw(Ar^(c)) of the aromatic vinyl polymer block Ar^(c) of the blockcopolymer C be substantially equal to at least one of the weight averagemolecular weight Mw(Ar1^(a)) of the aromatic vinyl polymer block Ar1^(a)having a relatively small weight average molecular weight of the blockcopolymer A and the weight average molecular weight Mw(Ar^(b)) of thearomatic vinyl polymer block Ar^(b) of the block copolymer B, and it ismore preferable that the Mw(Ar^(c)) be substantially equal to the bothof Mw (Ar1^(a)) and Mw(Ar^(b)).

The vinyl bond content of the conjugated diene polymer block D^(c) ofthe block copolymer C is 21% to 70% by mole, preferably 21% to 65% bymole, and more preferably 21% to 60% by mole. The vinyl bond content inthe above described range allows the crosslinking properties of theblock copolymer composition for a flexographic plate to be excellent,and thus clear images may be formed in a flexographic plate while thedurability of the image may be improved. In addition, it is preferablethat the vinyl bond content of the conjugated diene polymer block D^(c)of the block copolymer C be substantially equal to at least one of thevinyl bond content of the conjugated diene polymer block D^(a) of theblock copolymer A and the vinyl bond content of the conjugated dienepolymer block D^(b) of the block copolymer B, and it is more preferablethat the vinyl bond content of D^(c) be equal to both the vinyl bondcontent of D^(a) and D^(b).

The weight average molecular weight Mw(D^(c)) of the conjugated dienepolymer block D^(c) of the block copolymer C is not particularlylimited, but is usually 40,000 to 200,000, preferably 42,000 to 180,000,and more preferably 45,000 to 150,000. The Mw(D^(c)) is in the abovedescribed range, and thus the block copolymer composition for aflexographic plate to be obtained may have excellent rubber elasticity.In addition, it is preferable that the weight average molecular weightMw(D^(c)) of the conjugated diene polymer block D^(c) of the blockcopolymer C be substantially equal to at least one of the weight averagemolecular weight Mw(D^(a)) of the conjugated diene polymer block D^(a)of the block copolymer A and the weight average molecular weightMw(D^(b)) of the conjugated diene polymer block D^(b) of the blockcopolymer B, and it is more preferable that the Mw(D^(c)) besubstantially equal to the both of Mw(D^(a)) and Mw(D^(b)).

The content of the aromatic vinyl monomer unit relative to all themonomer units in the block copolymer C is note particularly limited, butis usually 10% to 35% by mass, preferably 12% to 32% by mass, and morepreferably 15% to 30% by mass. In addition, the content of the aromaticvinyl monomer unit relative to all the monomer units in the blockcopolymer C is preferably substantially equal to the content of thearomatic vinyl monomer unit relative to all the monomer units in theblock copolymer B.

The weight average molecular weight of the block copolymer C as a wholeis also not particularly limited, but is usually 46,000 to 200,000,preferably 50,000 to 180,000, and more preferably 55,000 to 160,000.

The molecular weight distribution that is expressed as the ratio Mw/Mnbetween the weight average molecular weight Mw and the number averagemolecular weight Mn of the various polymer blocks constituting each ofthe block copolymers A to C is not particularly limited, but themolecular weight distribution is, in each case, usually 1.1 or less, andpreferably 1.05 or less.

The mass ratio A/B of the block copolymer A with respect to the blockcopolymer B included in the block copolymer composition for aflexographic plate of the present invention is not particularly limited,but is preferably 36/64 to 85/15, more preferably 38/62 to 80/20, andparticularly preferably 40/60 to 75/25. The block copolymer A and theblock copolymer B are included in such a ratio, and thus the blockcopolymer composition for a flexographic plate to be obtained may beexcellent in abrasion resistance while maintaining sufficient rubberelasticity.

There are no particular limitations on the amount of the block copolymerC that can be included in the block copolymer composition forflexographic plate of the present invention. As a mass ratio C/(A+B),which is relative to the total mass of the block copolymer A and theblock copolymer B, the amount of the block copolymer C is preferably0/100 to 50/50, more preferably 5/95 to 40/60, and particularlypreferably 10/90 to 30/70. The block copolymer C is included in such aratio, and thus the block copolymer composition for a flexographic platemay be particularly well-balanced in abrasion resistance and rubberelasticity.

The block copolymer composition for a flexographic plate of the presentinvention may include just the block copolymers A to C as the polymercomponents, and may also include a polymer component other than theblock copolymers A to C. Examples of the polymer component other thanthe block copolymers A to C that can be included in the block copolymercomposition for a flexographic plate of the present invention may be anaromatic vinyl-conjugated diene-aromatic vinyl block copolymer otherthan the block copolymer A and the block copolymer B, an aromaticvinyl-conjugated diene block copolymer other than the block copolymer C,an aromatic vinyl homopolymer, a conjugated diene homopolymer, anaromatic vinyl-conjugated diene random copolymer, and branch-typepolymers of these, or a thermoplastic elastomer such as apolyurethane-based thermoplastic elastomer, a polyamide-basedthermoplastic elastomer, and a polyester-based thermoplastic elastomer,and a thermoplastic resin such as polyethylene, polypropylene,polyvinylchloride, an acrylonitrile-styrene copolymer, anacrylonitrile-butadiene-styrene copolymer, and polyphenylene ether. Inthe block copolymer composition for a flexographic plate of the presentinvention, the content of the polymer component other than the blockcopolymers A to C relative to all the polymer components is, preferably20% by mass or less, and more preferably 10% by mass or less.

In the block copolymer composition for a flexographic plate of thepresent invention, the content of the aromatic vinyl monomer unitrelative to all the monomer units that constitutes the polymer component(in the following descriptions, may be referred to as “overall contentof aromatic vinyl monomer unit”) is, 18% to 70% by mass, preferably 20%to 70% by mass, more preferably 22% to 60% by mass, and particularlypreferably 25% to 50% by mass. If the overall content of aromatic vinylmonomer unit is too small, there is a risk that the block copolymercomposition for a flexographic plate may be inferior in abrasionresistance and ink-selling resistance, and if the overall content ofaromatic vinyl monomer unit is too large, there is a risk that the blockcopolymer composition for a flexographic plate may lose rubberelasticity required as a flexographic plate. The overall content ofaromatic vinyl monomer unit can be easily adjusted by considering eachcontent of the aromatic vinyl monomer unit of the block copolymers A toC and the polymer component other than those that constitute the blockcopolymer composition for a flexographic plate, and adjusting theblending amount of those. Incidentally, in a case all of the polymercomponents that constitute the block copolymer composition for aflexographic plate are composed only of aromatic vinyl monomer units andconjugated diene monomer units, when the polymer components of the blockcopolymer composition for a flexographic plate are subjected to ozonedecomposition and then to reduction by lithium aluminum hydrideaccording to the method described in Rubber Chem. Technol., 45, 1295(1972), the conjugated diene monomer unit moieties are decomposed andonly the aromatic vinyl monomer unit moieties can be extracted.Therefore, the overall content of aromatic vinyl monomer units can bemeasured easily.

The weight average molecular weight of the overall polymer componentsthat constitute the block copolymer composition for a flexographic plateof the present invention is not particularly limited, but is usually50,000 to 500,000, preferably 60,000 to 450,000, and more preferably70,000 to 400,000. In addition, the molecular weight distributionrepresented by the ratio Mw/Mn of the weight average molecular weight(Mw) and the number average molecular weight (Mn) of the overall polymercomponents that constitute the block copolymer composition for aflexographic plate of the present invention is not particularly limited,but is usually 1.01 to 10, preferably 1.03 to 5, and more preferably1.05 to 3.

The type A hardness of the block copolymer composition for aflexographic plate of the present invention is 25 to 65, preferably 26to 65, and more preferably 27 to 65. The type A hardness is in the abovedescribed range, and thus the block copolymer composition for aflexographic plate may maintain rubber elasticity even when theproportion of the aromatic vinyl monomer unit therein is increased tohave excellent abrasion resistance.

The type A hardness here is a value measured with a durometer hardnesstester (type A) in accordance with JIS K6253.

In addition, the anisotropic index of the block copolymer compositionfor a flexographic plate of the present invention is 2.0 or less,preferably 1.8 or less, and more preferably 1.5 or less. Theflexographic plate that does not easily express anisotropy but has highisotropy and does not easily cause printing defect may be obtained withthe isotropic index in the above described range, even when a formingmethod that easily cause molecular orientation such as an extrusionmolding method is used when the flexographic plate is produced.

The isotropic index here is a value obtained from the rate of “a tensileelastic modulus in the melt flow direction/a tensile elastic modulus inthe vertical direction to melt flows” of two of a sheet produced bymelting and extrusion molding the block copolymer composition for aflexographic plate, one of which tensile elastic modulus along with themelt flow direction during molding is measured, and the other of whichthe tensile elastic modulus along with the vertical direction to meltflows during molding is measured. When the rate of “a tensile elasticmodulus in the melt flow direction/a tensile elastic modulus in thevertical direction to melt flows” is closer to 1, the anisotropy issmaller and isotropy is more excellent.

There are no particular limitations on the method for obtaining theblock copolymer composition for a flexographic plate of the presentinvention. For example, the composition may be produced in a manner suchthat the block copolymer A and the block copolymer B may be respectivelyproduced in accordance with a conventional production method of a blockcopolymer separately, and the block copolymer C and an additionalpolymer component may be blended therewith as required, and then mixedin accordance with a usual method such as kneading and solution mixing.However, from the aspect of obtaining the block copolymer compositionhaving particularly desirable configuration with better productivity, aproduction method described later is suitable.

The above described block copolymer composition for a flexographic plateof the present invention is provided with abrasion resistance, which ishighly superior to conventional polymer compositions for a flexographicplate, while maintaining sufficient rubber elasticity, excellentcross-linking properties, and small anisotropy. Accordingly, with theuse of the block copolymer composition for a flexographic plate of thepresent invention, a flexographic plate having high definition printingproperties, excellent ink-swelling resistance, and highly balancedabrasion resistance and flexibility may be obtained. There are noparticular limitations on the way of obtaining a flexographic plate withthe use of the block copolymer composition for a flexographic plate ofthe present invention, but a general way of obtaining a flexographicplate is by making the composition to be photosensitive and molding thephotosensitive composition to be in a sheet shape, then exposing thesheet to light.

B. Method for Producing Block Copolymer Composition for FlexographicPlate

The block copolymer composition for a flexographic plate of the presentinvention is preferably produced using a production method including thefollowing steps (1) to (5).

(1): A first step of forming an aromatic vinyl polymer including anactive terminal by polymerizing an aromatic vinyl monomer using apolymerization initiator in a solvent;

(2): A second step of forming an aromatic vinyl-conjugated diene blockcopolymer including an active terminal by adding a conjugated dienemonomer to a solution obtained in the first step for polymerization;

(3): A third step of forming a block copolymer B by adding a couplingagent to a solution obtained in the second step, the coupling agent inan amount the functional group relative to the active terminal is lessthan one equimolar amount;

(4): A fourth step of forming a block copolymer A by adding an aromaticvinyl monomer to a solution obtained in the third step forpolymerization; and

(5): A fifth step of collecting the block copolymer composition for theflexographic plate from a solution obtained in the fourth step.

According to the production method described above, since the blockcopolymer A and the block copolymer B can be continuously obtained inthe same reaction vessel, an intended block copolymer composition for aflexographic plate can be obtained with extremely excellent productivityas compared with the case of individually producing the respective blockcopolymers and mixing them. On top of that, the composition to beobtained may have particularly desirable balance of the weight averagemolecular weight of each of the polymer blocks in each of the blockcopolymers as the block copolymer composition for a flexographic plate;thus, the block copolymer composition for a flexographic plateparticularly excellent in balance of abrasion resistance and rubberelasticity can be obtained.

Above all, as described in the section “A. Block copolymer compositionfor flexographic plate” above, the coupling agent to be added in thethird step is preferably a compound including two or more of at leastone kind of functional group selected from an alkoxyl group, an estergroup and an epoxy group in one molecule. In this case, it is preferablethat the reactions in at least the second step, the third step, and thefourth step are conducted under the presence of a Lewis base compound,and the amount of the Lewis base compound is in a specified range. Theproduction method like above is a suitable method for producing theabove described block copolymer composition for a flexographic plate,wherein the block copolymer B in the block copolymer composition for aflexographic plate is obtained by using, as a coupling agent, a compoundincluding two or more of at least one kind of functional group selectedfrom an alkoxyl group, an ester group and an epoxy group in onemolecule.

Each step of the method for producing the block copolymer composition ofthe present invention is hereinafter described.

1. First Step

In the first step, an aromatic vinyl polymer including an activeterminal is formed by polymerizing an aromatic vinyl monomer using apolymerization initiator in a solvent.

As the polymerization initiator used in the first step, a compoundgenerally known to have an anion polymerization activity to an aromaticvinyl monomer and a conjugated diene monomer, such as an organic alkalimetal compound, an organic alkali earth metal compound, and an organicrare earth metal compound of lanthanoid series may be used. As theorganic alkali metal compound, an organic lithium compound including oneor more of a lithium atom in one molecule is particularly suitably used.Specific examples thereof may include an organic monolithium compoundsuch as ethyllithium, n-propyllithium, isopropyllithium, n-butyllithium,sec-butyllithium, t-butyllithium, hexyllithium, phenyllithium,stilbenelithium, dialkylaminolithium, diphenylaminolithium, andditrimethylsilylaminolithium; an organic dilithium compound such asmethylenedilithium, tetramethylenedilithium, hexamethylenedilithium,isoprenyldilithium, and 1,4-dilithio-ethylcyclohexane; and an organictrilithium compound such as 1,3,5-trilithiobenzene. Among those, theorganic monolithium compound is particularly suitably used.

Examples of the organic alkali earth metal compound used as thepolymerization initiator may include n-butylmagnesiumbromide,n-hexylmagnesiumbromide, ethoxycalcium, calcium stearate,t-butoxystrontium, ethoxybarium, isopropoxybarium, ethylmercaptobarium,t-butoxybarium, phenoxybarium, diethylaminobarium, barium stearate, andethylbarium. Furthermore, specific examples of other polymerizationinitiators include compounds that form a uniform system in organicsolvent and have living polymerizability, such as composite catalystsformed from lanthanoid-based rare earth metal compounds includingneodymium, samarium and gadolinium/alkylaluminum/alkylaluminumhalide/alkylaluminum hydride; and metallocene type catalysts includingtitanium, vanadium, samarium, and gadolinium. Incidentally, thesepolymerization initiators may be used singly, or two or more kindsthereof may be used in mixture.

The amount of use of the polymerization initiator may be determined inaccordance with the molecular weights of the various intended blockcopolymers and are not particularly limited. However, the amount of useis usually 0.01 millimole to 20 millimoles, preferably 0.05 millimole to15 millimoles, and more preferably 0.1 millimole to 10 millimoles, per100 g of all the monomers used.

The solvent used for the polymerization is not particularly limited aslong as it is inert to the polymerization initiator, and for example,chain-like hydrocarbon solvents, cyclic hydrocarbon solvents, or solventmixtures thereof are used. Examples of the chain-like hydrocarbonsolvents include chain-like alkanes and alkenes having 4 to 6 carbonatoms, such as n-butane, isobutane, 1-butene, isobutylene,trans-2-butene, cis-2-butene, 1-pentene, trans-2-pentene, cis-2-pentene,n-pentane, isopentane, neo-pentane, and n-hexane. Specific examples ofthe cyclic hydrocarbon solvents include aromatic compounds such asbenzene, toluene, and xylene; and alicyclic hydrocarbon compounds suchas cyclopentane and cyclohexane. These solvents may be used singly, ortwo or more kinds thereof may be used in mixture.

The amount of the solvent used for polymerization is not particularlylimited, but the amount is set such that the total concentration of theblock copolymers in the solution obtainable after the polymerizationreaction would be usually 5% to 60% by mass, preferably 10% to 55% bymass, and more preferably 20% to 50% by mass.

On the occasion of obtaining the block copolymer composition for aflexographic plate, in order to control the structures of the variouspolymer blocks of the various block copolymers, a Lewis base compoundmay be added to the reactor used for the polymerization. Examples ofthis Lewis base compound include ethers such as tetrahydrofuran, diethylether, dioxane, ethylene glycol dimethyl ether, ethylene glycol dibutylether, diethylene glycol dimethyl ether, and diethylene glycol dibutylether; tertiary amines such as tetramethylethylenediamine,trimethylamine, triethylamine, pyridine, and quinuclidine; alkali metalalkoxides such as potassium t-amyl oxide and potassium t-butyl oxide;and phosphines such as triphenylphosphine. These Lewis base compoundsare used singly or in combination of two or more kinds thereof, and areappropriately selected to the extent that the purposes of the presentinvention are not impaired.

Furthermore, the timing for adding the Lewis base compound at the timeof the polymerization reaction is not particularly limited, and may beappropriately determined in accordance with the intended structures ofthe various block copolymers. For example, the Lewis base compound maybe added in advance before polymerization is initiated, may be addedafter a portion of the polymer blocks have been polymerized, or may beadded in advance before polymerization is initiated and then furtheradded after a portion of the polymer blocks have been polymerized.

The polymerization reaction temperature is usually 10° C. to 150° C.,preferably 30° C. to 130° C., and more preferably 40° C. to 90° C. Thetime required for polymerization may vary with the conditions, but thetime for the polymerization reaction is usually 48 hours or less, andpreferably 0.5 hour to 10 hours. The polymerization pressure is notparticularly limited, and polymerization may be carried out in apressure range sufficient for maintaining the monomers and the solventin a liquid state in the polymerization temperature range mentionedabove.

When an aromatic vinyl monomer is polymerized in a solvent using apolymerization initiator under the conditions such as described above, asolution containing an aromatic vinyl polymer including an activeterminal can be obtained. This aromatic vinyl polymer including anactive terminal constitutes the aromatic vinyl polymer block (Ar1^(a))having a relatively small weight average molecular weight of the blockcopolymer A, and the aromatic vinyl polymer block (Ar^(b)) of the blockcopolymer B, those constitute the block copolymer composition.Therefore, the amount of the aromatic vinyl monomer used at this time isdetermined depending on the intended weight average molecular weights ofthese polymer blocks.

2. Second Step

In the second step, an aromatic vinyl-conjugated diene block copolymerincluding an active terminal is formed by adding a conjugated dienemonomer to a solution obtained in the first step for polymerization.

Through the addition of a conjugated diene monomer, a conjugated dienepolymer chain is formed from the active terminal, and thus a solutioncontaining an aromatic vinyl-conjugated diene block copolymer(diblock)including an active terminal is obtained. The amount of the conjugateddiene monomer used at this time is determined such that the resultingconjugated diene polymer chain would have the intended weight averagemolecular weight of the conjugated diene polymer block (D^(b)) of theblock copolymer B.

In the second step, it is preferable that the polymerization isconducted under the presence of a Lewis base compound. In this case, theamount of the Lewis base compound per 1 mole of the active terminal inthe polymerization initiator is preferably 0.1 mole to 50 moles, morepreferably 0.1 mole to 45 moles, and particularly preferably 0.1 mole to40 moles. The polymerization of the conjugated diene monomer isconducted under the presence of the Lewis base compound, and thus thestructure of the conjugated diene polymer block to be obtained may becontrolled. On this occasion, the vinyl bond content in the conjugateddiene polymer block to be obtained may be in a specified range, which isrelatively high, when the amount of the Lewis base compound is in theabove described range, which is relatively much. Thereby, crosslinkingproperties of the block copolymer composition for a flexographic platemay be improved, and thus a clear image can be formed in theflexographic plate while the durability of the image can be improved.

In addition, as described above, in the third step, when a compoundincluding two or more of at least one kind of functional group selectedfrom an alkoxyl group, an ester group and an epoxy group in one moleculeis used as a coupling agent, it is preferable that the reactions in atleast the second step, the third step, and the fourth step are conductedunder the presence of the Lewis base compound. In this case, the timingof adding the Lewis base compound is not particularly limited as long asthe reactions in the second step, the third step, and the fourth stepcan be conducted under the presence of the Lewis base compound. Forexample, the Lewis base compound may be added in the first step inadvance before starting the polymerization, the Lewis base compound maybe added in the second step after the first step, or the Lewis basecompound may be added in the first step before starting thepolymerization as well as further added in the second step. Also, theLewis base compound may be further added in the third step, and theLewis base compound may be further added in the fourth step.

In the case of adding the Lewis base compound further in the third stepand the fourth step, the total amount of the Lewis base compound to beadded in all the steps may be in the above described range. In thiscase, it is preferable that the amount of the Lewis base compound to beadded until the second step is 0.1 mole or more per 1 mole of the activeterminal in the polymerization initiator. As described above, the reasontherefor is that the vinyl bond content in the conjugated diene polymerblock to be obtained may be in a specified range, which is relativelyhigh, when the amount of the Lewis base compound in the second step isin the above described range, which is relatively much.

3. Third Step

In the third step, a block copolymer B is formed by adding a couplingagent to a solution including the aromatic vinyl-conjugated diene blockcopolymer including the active terminal obtained in the second step, inan amount the functional group relative to the active terminal is lessthan one equimolar amount.

When a coupling agent is added to a solution including the aromaticvinyl-conjugated diene block copolymer (diblock) including the activeterminal, in an amount the functional group relative to the activeterminal is less than one equimolar amount, in partial copolymer amongthe aromatic vinyl-conjugated diene block copolymer (diblock) includingthe active terminal, conjugated diene polymer blocks are bonded to eachother interposing the residue of the coupling agent; as a result, theblock copolymer B of the block copolymer composition is formed. Then, apart of the rest of the aromatic vinyl-conjugated diene block copolymer(diblock) including the active terminal is left unreacted in thesolution.

Examples of the coupling agent is as described in the section “A. Blockcopolymer composition for flexographic plate” above. In addition, theamount of the coupling agent to be added is not particularly limited aslong as it is an amount determined in accordance with the ratio of theblock copolymer A and the block copolymer B that constitute the blockcopolymer composition, and is an amount in which the functional group ofthe coupling agent relative to the active terminal of the polymer isless than one equimolar amount; however, the amount is usually such thatthe functional group of the coupling agent relative to the activeterminal of the polymer is in the range of 0.10 equimolar to 0.90equimolar, and preferably in the range of 0.15 equimolar to 0.70equimolar. Incidentally, the conditions for the coupling reaction is notparticularly limited, and is usually selected from the range of theaforementioned polymerization reaction conditions.

Furthermore, as described above, when a compound including two or moreof at least one kind of functional group selected from an alkoxyl group,an ester group and an epoxy group in one molecule is used as thecoupling agent, it is preferable that the reaction in the third step isconducted under the presence of the Lewis base compound.

Incidentally, with the above production method, there may be cases wherea polymer component comprising an aromatic vinyl polymer block and aconjugated diene polymer block, that is different from all of the blockcopolymers A to C, is included in the block copolymer composition for aflexographic plate. For example, when an alkoxysilane compound includingtwo or more of an alkoxy group directly bonded to a silicon atom in onemolecule is used as the coupling agent, in this polymer component, thecontent of the aromatic vinyl monomer unit relative to all the monomerunits is approximately in the middle level between that of the blockcopolymer A and that of the block copolymer B. In addition, the weightaverage molecular weight of the polymer component is approximately onetime to three times of the weight average molecular weight of the blockcopolymer A. The structure of this polymer component is not alwaysclear, but is presumed to be a block copolymer D represented by thebelow general formula (D) that can be generated from a mechanism suchthat the aromatic vinyl-conjugated diene block copolymer including anactive terminal does not react with all the alkoxysilyl groups in thecoupling agent and a part of the alkoxysilyl groups remains unreactedwhen a coupling block copolymer is formed, and then when the aromaticvinyl-conjugated diene-aromatic vinyl block copolymer including anactive terminal is formed, that active terminal reacts with theunreacted alkoxysilyl group in the coupling agent.

(Ar^(b)-D^(b))_(n-1)-X-Ar2^(a)-D^(a)-Ar1^(a)  (D)

In the general formula (D), Ar2^(a), Ar^(b), and D^(b) represent thesame as those in the general formula (A) and the general formula (B), Xrepresents the residue of coupling agent, and n represents an integer of2 or more.

When the polymer component presumed to be the block copolymer D asdescribed above is included, the block copolymer composition for aflexographic plate may, in particular, have excellent balance ofabrasion resistance and rubber elasticity and excellent isotropy thatdoes not easily express anisotropy even when a forming method thateasily cause molecular orientation such as an extrusion molding methodis used. Therefore, when the block copolymer composition for aflexographic plate of the present invention is produced by the abovedescribed production method, it is particularly preferable that analkoxysilane compound including two or more of an alkoxy group directlybonded to a silicon atom per one molecule is used as the coupling agent.

4. Fourth Step

In the fourth step, a block copolymer A is formed by adding an aromaticvinyl monomer to a solution obtained in the third step forpolymerization.

When an aromatic vinyl monomer is added to the solution obtained in thethird step, an aromatic vinyl polymer chain is formed from the terminalof the aromatic vinyl-conjugated diene block copolymer (diblock)including an active terminal that remained unreacted with the couplingagent. This aromatic vinyl polymer chain constitutes the aromatic vinylpolymer block (Ar2^(a)) having a relatively large weight averagemolecular weight of the block copolymer A that constitutes the blockcopolymer composition for a flexographic plate. Therefore, the amount ofthe aromatic vinyl monomer used at this time is determined in accordancewith the intended weight average molecular weight of the aromatic vinylpolymer block (Ar2^(a)). Through this process for adding an aromaticvinyl monomer, an asymmetric aromatic vinyl-conjugated diene-aromaticvinyl block copolymer that constitutes the block copolymer A is formed,and as a result, a solution containing the block copolymer A and theblock copolymer B is obtained. Incidentally, a conjugated diene monomermay be added to the solution containing an aromatic vinyl-conjugateddiene block copolymer (diblock) including an active terminal that didnot react with the coupling agent, before this step of adding anaromatic vinyl monomer. As such, when a conjugated diene monomer isadded, the weight average molecular weight of the conjugated dienepolymer block (D^(a)) of the block copolymer A can be made largercompared to the case of not adding the conjugated diene monomer.

As described above, when a compound including two or more of at leastone kind of functional group selected from an alkoxyl group, an estergroup and an epoxy group in one molecule is used as the coupling agentin the third step, it is preferable that the polymerization in thefourth step is conducted under the presence of the Lewis base compound,and the amount of the Lewis base compound is in a specified range. Whena compound including two or more of at least one kind of functionalgroup selected from an alkoxyl group, an ester group, and an epoxy groupin one molecule is used as the coupling agent in the third step, theremay be cases where a polymer component comprising an aromatic vinylpolymer block and a conjugated diene polymer block, that is differentfrom all of the block copolymers A to C, is formed. Particularly when analkoxysilane compound including two or more of an alkoxy group directlybonded to a silicon atom in one molecule is used as the coupling agentin the third step, as described above, there may be cases where apolymer component presumed to be the block copolymer D different fromall of the block copolymers A to C is formed. In this case, such a sidereaction may be inhibited by conducting the polymerization of thearomatic vinyl monomer under the presence of the Lewis base compound. Onthis occasion, the side reaction can be effectively inhibited by settingthe amount of the Lewis base compound to be in a specified range, thatis relatively much. Accordingly, the structure of the block copolymer tobe obtained can be controlled while the side reaction can be effectivelyinhibited, and thus the block copolymer having a specified structure canbe stably produced.

5. Fifth Step

In the fifth step, the block copolymer composition for the flexographicplate is collected from a solution obtained in the fourth step.

The method for collection may be carried out according to a conventionalmethod, and there are no particular limitations. For example, thepolymer components can be collected by adding, if necessary, apolymerization terminator such as water, methanol, ethanol, propanol,hydrochloric acid or citric acid after completion of the reaction,further adding additives such as an antioxidant as necessary, andapplying a known method such as a direct drying method or steamstripping to the solution. When the block copolymer composition iscollected as slurry by a method such as applying steam stripping, theblock copolymer composition are dehydrated using an arbitrary dehydratorsuch as an extruder type squeezer to obtain a crumb having a watercontent of a predetermined value or less, and the crumb may be driedusing an arbitrary dryer such as a band dryer or an expansion extrusiondryer. The block copolymer composition for a flexographic plate obtainedas described above may be processed into a shape such as pelletsaccording to a conventional method, and then may be supplied to the use.

6. Sixth Step

In the present invention, a sixth step of forming a block copolymer C byadding a polymerization terminator to the solution including an aromaticvinyl-conjugated diene block copolymer including an active terminal thatdid not react with the coupling agent obtained in the third step, in anamount less than equal amount of the active terminal, may be conductedafter the third step and before the fourth step.

Addition of the polymerization terminator in this manner woulddeactivate the active terminal of the aromatic vinyl-conjugated dieneblock copolymer (diblock), and thus the aromatic vinyl-conjugated dieneblock copolymer (diblock) to be obtained thereby may be included in theblock copolymer composition for a flexographic plate as the blockcopolymer C.

As the polymerization terminator, for example, a substance such aswater, methanol, ethanol, propanol, hydrochloric acid, and citric acidmay be used.

Incidentally, when the sixth step is arranged, the total amount of thefunctional group in the coupling agent and the polymerization terminatorrelative to the active terminal of the aromatic vinyl-conjugated dieneblock copolymer to be added needs to be an amount less than oneequimolar amount.

C. Photosensitive Composition for Flexographic Plate

The photosensitive composition for a flexographic plate of the presentinvention comprises the above described block copolymer composition fora flexographic plate, an ethyleny unsaturated compound having amolecular weight of 5,000 or less, and a photopolymerization initiator.

The blending amount of the block copolymer composition for aflexographic plate relative to the total amount of the block copolymercomposition for a flexographic plate and the ethyleny unsaturatedcompound is preferably 40% by mass to 95% by mass, and more preferably50% by mass to 95% by mass.

Examples of the ethyleny unsaturated compound having a molecular weightof 5,000 or less may include diacrylate or dimethacrylate of divalentalcohol such as ethyleneglycol, diethyleneglycol, propyleneglycol,dipropyleneglycol, polyethyleneglycol, polypropyleneglycol,1,4-butandiol, and 1,6-hexandiol; triacrylate or trimethacrylate oftrimethylolpropane; tetraacrylate or tetramethacrylate ofpentaerythritol; N,N′-(hexamethylene)bis(acrylamide),N,N′-(hexamethylene)bis(methacrylamide), diacetone acrylamide, diacetonemethacrylamide, styrene, vinyltoluene, divinylbenzene, diallylphthalate, and triallyl cyanurate. These may be used singly, or two ormore kinds thereof may be used in combination.

The blending amount of the ethyleny unsaturated compound relative to thetotal amount of the block copolymer composition for a flexographic plateand the ethyleny unsaturated compound is preferably 5% by mass to 60% bymass, and more preferably 5% by mass to 50% by mass.

The total amount of the block copolymer composition for a flexographicplate and the ethyleny unsaturated compound relative to overall amountof the photosensitive compound for a flexographic plate is preferably50% by mass or more, more preferably 60% by mass or more, andparticularly preferably 70% by mass or more.

Examples of the photopolymerization initiator may includemethylhydroquinone, benzophenone, benzoin, benzoin methyl ether, benzoinethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, α-methylbenzoin, α-methylbenzoin methyl ether, α-methoxybenzoin methyl ether,benzoin phenyl ether, α-t-butylbenzoin, anthraquinone,benzanthraquinone, 2-ethylanthraquinone, 2-chloranthraquinone,2-2′-dimethoxy diphenyl acetophenone, 2,2-diethoxyphenylacetophenone,2,2-diethoxyacetophenone, and pivaloin. These may be used singly, or twoor more kinds thereof may be used in combination. The blending amount ofthe photopolymerization initiator relative to the total amount of theblock copolymer composition for a flexographic plate and the ethylenyunsaturated compound is preferably 0.1% by mass to 5% by mass.

In the present invention, a component other than the above may also beadded to the photosensitive composition for a flexographic plate asrequired. Examples of such a component may include a plasticizer, athermopolymerization inhibitor, an antioxidant, an antiozonant, a dye, apigment, a filler, an additive that exhibits photochromism, a reductant,an agent that improves relief structure, a crosslinking agent, a flowimprover, and a mold releasing agent.

The plasticizer is usually used for purposes such as to easily produceand form the photosensitive composition for a flexographic plate, topromote the removal of nonexposed portion, and to adjust the hardness ofcured exposed portion. Examples of the plasticizer may include ahydrocarbon oil such as a naphthenic oil and a paraffin oil; liquid1,2-polybutadiene, liquid 1,4-polybutadiene, and the hydroxide orcarboxylate of these; a liquid acrylonitrile-butadiene copolymer and thecarboxylate thereof; a liquid styrene-butadiene copolymer and thecarboxylate thereof; polystyrene having low molecular weight which isthe molecular weight of 3,000 or less, an α-methylstyrene-vinyltoluenecopolymer, a petroleum resin, a polyacrylate resin, a polyester resin,and a polyterpene resin. These may be used singly, or two or more kindsthereof may be used in combination. The plasticizer is usually added tothe photosensitive composition for a flexographic plate in the range of2% by mass to 50% by mass in accordance with the intendedcharacteristics.

The thermopolymerization inhibitor is used for the purpose of inhibitingunintended thermopolymerization of the ethyleny unsaturated compoundwhen the photosensitive composition for a flexographic plate isproduced. Examples of the thermopolymerization inhibitor may includephenols such as hydroquinone, p-methoxyphenol, p-t-butylcatechol,2,6-di-t-butyl-p-cresol, and pyrogallol; quinones such as benzoquinone,p-toluquinone, and p-xyloquinone; and amines such asphenyl-a-naphthylamine. These may be used singly, or two or more kindsthereof may be used in combination. The amount of use of thephotopolymerization inhibitor is usually 0.001% by mass to 2% by mass inthe photosensitive composition for a flexographic plate.

There are no particular limitations on the method for producing thephotosensitive composition for a flexographic plate of the presentinvention. For example, the composition may be produced by kneading theconstituent components of the composition with means such as a kneader,a roll mill, Banbury, and a uniaxial or multiaxial extruder. Theobtained composition is usually molded into a sheet shape having adesired thickness, using a molding machine such as a uniaxial ormultiaxial extruder, a compression molding machine, and a calendarmolding machine. Incidentally, when the uniaxial or multiaxial extruderis used, the production of the photosensitive composition for aflexographic plate and molding into a sheet shape can be simultaneouslyconducted. Further, the photosensitive composition for a flexographicplate in a sheet shape can also be produced in a manner such that theconstituent components of the photosensitive composition for aflexographic plate are dissolved in an appropriate solvent such aschloroform, carbon tetrachloride, trichloroethane, diethyl ketone,methyl ethyl ketone, benzene, toluene, and tetrahydrofuran, and thenthis solution is poured to a frame to vaporize the solvent.

The thickness of the sheet is usually 0.1 mm to 20 mm, and preferably 1mm to 10 mm.

To the photosensitive composition for a flexographic plate in the sheetshape, a transparent sheet or a film formed of a resin such aspolypropylene, polyethylene, and polyethylene terephthalate may bearranged on its surface as a base sheet layer or a protective film layerin order to prevent the contamination and damage of the photosensitivecomposition for a flexographic plate during its storage or operation.

A thin covering material layer with high flexibility may be arranged onthe surface of the photosensitive composition for a flexographic platein the sheet shape in order to restrain the adhesiveness of the surfaceof the composition and to enable the reuse of negative film after lightirradiation. In this case, after the completion of the light exposure ofthe photosensitive composition for a flexographic plate, the coveringmaterial layer should be removed at the time of removing the unexposedportion with the solvent together. As the covering material layer,typically, a material such as a soluble polyamide and a cellulosederivative are frequently used.

D. Flexographic Plate

The flexographic plate of the present invention is obtained by exposingthe photosensitive composition for a flexographic plate of the presentinvention to light.

The production of the flexographic plate is usually conducted inaccordance with the following steps:

(i): Light is irradiated from the base sheet side of the multilayeredsheet formed from a protective film, the layer of photosensitivecomposition for a flexographic plate in the sheet shape, and the basesheet, so as to cure the layer of photosensitive composition for aflexographic plate until a specified thickness;(ii): The protective film is peeled off, a negative film is adhered, andlight having a wavelength of 230 nm to 450 nm or preferably 350 nm to450 nm is irradiated from above the negative film, and thereby the layerof the photosensitive composition for a flexographic plate is exposed tolight. With this exposure, a portion of the layer of the photosensitivecomposition for a flexographic plate where the light transmitted iscured;(iii): The portion of the layer of the photosensitive composition for aflexographic plate not exposed to light uncured is removed(development);(iv): The uncured portion is removed in (iii) typically using a solvent,so the solvent remaining in the flexographic plate is dried;(v): As desired, postexposure is conducted.

In the development (removal of the unexposed portion) step of (iii), asolvent is typically used. Examples of the solvent may include analiphatic hydrocarbon or aromatic hydrocarbon such as n-hexane,n-heptane, octane, petroleum ether, naphtha, limonene, terpene, toluene,xylene, ethylbenzene, and isopropyl benzene; ketones such as acetone andmethyl ethyl ketone; ethers such as di-n-butylether, anddi-t-butylether; esters such as methyl acetate and ethyl acetate; andhalogenated hydrocarbon such as methylene chloride, chloroform,trichloroethane, tetrachloroethylene, dichlorotetrafluoroethane, andtrichlorotrifluoroethane. These may be used singly, or two kinds or morethereof may be used in combination. Further, to the solvent, a desiredamount of alcohol such as methanol, ethanol, isopropanol, and n-butanolmay be added and used. Incidentally, the development may be expedited byadding a mechanical force using means such as a brush under the presenceof the solvent.

The flexographic plate of the present invention has sufficientflexibility, highly excellent abrasion resistance, and excellentink-swelling resistance. Accordingly, the use of the flexographic plateof the present invention enables repeating print in multiple of timeseven under strict conditions, with excellent ink transfer duringprinting, and flexographic printing with excellent image quality.Incidentally, examples of the subject for the application offlexographic printing are various materials such as paper, cardboard,wood, metal, a polyethylene film, a polyethylene sheet, a polypropylenefilm, and polypropylene sheet.

The present invention is not limited to the embodiments. The embodimentsare exemplifications, and any other variations are intended to beincluded in the technical scope of the present invention if they havesubstantially the same constitution as the technical idea described inthe claim of the present invention and offer similar operation andeffect thereto.

EXAMPLES

Hereinafter, the present invention will be more specifically describedby way of Examples and Comparative Examples. Incidentally, unlessparticularly stated otherwise, parts and percentage (%) in the variousExamples are on a mass basis.

Various measurements were carried out by the following methods.

[Weight Average Molecular Weight of Block Copolymer and Block CopolymerComposition]

The weight average molecular weight was determined as a molecular weightcalculated relative to polystyrene standards, by high performance liquidchromatography using tetrahydrofuran as a carrier at a flow rate of 0.35ml/min. The measurement was carried out using an HLC™-8220 manufacturedby Tosoh Corp. as an apparatus, with three connected columns of SHODEX™KF-404HQ manufactured by Showa Denko K.K. (column temperature 40° C.),and a differential refractometer and an ultraviolet detector asdetectors, and using twelve samples of polystyrene standards (from 500to 3,000,000) manufactured by Polymer Laboratories, Ltd. for thecalibration of the molecular weight.

[Mass Ratio of Various Block Copolymers]

The mass ratio was determined from the area ratio of peaks correspondingto the various block copolymers in the charts obtained by highperformance liquid chromatography as described above.

[Weight Average Molecular Weight of Styrene Polymer Block]

A block copolymer was caused to react with ozone according to the methoddescribed in Rubber Chem. Technol., 45, 1295 (1972), and was reducedusing lithium aluminum hydride, and thereby an isoprene polymer block ofthe block copolymer was decomposed. Specifically, the process wascarried out by the following procedure. That is, 300 mg of a sample wasdissolved in a reaction vessel containing 100 ml of dichloromethane thathad been treated with a molecular sieve. This reaction vessel was placedin a cooling tank, the temperature was set to −25° C., and then ozonegenerated by an ozone generator was introduced into the reaction vesselwhile oxygen was allowed to flow thereinto at a flow rate of 170 ml/min.After 30 minutes from the initiation of the reaction, it was checkedwhether the reaction had ended by introducing the gas discharged outfrom the reaction vessel into an aqueous solution of potassium iodide.Subsequently, 50 ml of diethyl ether and 470 mg of lithium aluminumhydride were introduced into another reaction vessel that had beenpurged with nitrogen, and while the reaction vessel was cooled with icewater, the solution that had reacted with ozone was slowly addeddropwise to this reaction vessel. Then, the reaction vessel was placedin a water bath to raise the temperature slowly, and the reactionsolution was refluxed at 40° C. for 30 minutes. Thereafter, dilutehydrochloric acid was added dropwise in small amounts while the solutionwas stirred, and dropwise addition was continued until generation ofhydrogen was almost not recognized. After this reaction, the solidproduct generated in the solution was separated by filtration, and thesolid product was extracted with 100 ml of diethyl ether for 10 minutes.This extract was combined with the filtrate obtained at the time ofseparation by filtration, and the solvent was distilled off. Thus, asolid sample was obtained. For the sample obtained as such, the weightaverage molecular weight was measured according to the method formeasuring the weight average molecular weight described above, and thevalue was designated as the weight average molecular weight of thestyrene polymer block.

[Weight Average Molecular Weight of Conjugated Diene Polymer Block]

The weight average molecular weight of the corresponding styrene polymerblock was subtracted from the weight average molecular weight of eachblock copolymer determined as described above, and the weight averagemolecular weight of the conjugated diene polymer block (isoprene polymerblock or butadiene polymer block) was determined based on thecalculation value.

[Styrene Unit Content of Block Copolymer]

The styrene unit content was determined based on the ratio between thedetection intensities of a differential refractometer and an ultravioletdetector in the analysis by high performance liquid chromatographydescribed above. Incidentally, copolymers having different styrene unitcontents were prepared, and a calibration curve was produced usingthose.

[Styrene Unit Content of Block Copolymer Composition]

The content was determined based on a proton NMR analysis.

[Vinyl Bond Content of Conjugated Diene Polymer Block]

The content was determined based on a proton NMR analysis.

[Type A Hardness of Block Copolymer Composition]

The type A hardness was measured with a durometer hardness tester (typeA) in accordance with JIS K6253.

[Anisotropic Index of Block Copolymer Composition]

The block copolymer composition for a flexographic plate was heated at150° C. and melted, and continuously extruded using a biaxial extruderwith T-dye installed, so as to be molded into a sheet having a thicknessof 2 mm. The detailed conditions for molding the sheet were as follows:

Composition treatment speed: 25 kg/hr;

Unloading speed: 1.0 m/min;

Extruder temperature: Input port 140° C., T-dye adjusted to 160° C.;

Screw: Full-flight;

Extruder L/D: 20; and

T-dye: Width 200 mm, lip 2.5 mm.

Two pieces of the obtained sheet were used, and the tensile elasticmodulus of the one was measured along with the melt flow directionduring molding, and the tensile elastic modulus of the other wasmeasured along with the vertical direction to the melt flow duringmolding. The measurement procedures were as follows. The sheet wasexpanded to 100% with the tension rate of 300 mm/min using TENSILONuniversal testing instrument RTC-1210 from ORIENTEC Co., LTD., and thetension stress at the expansion of 100% during the process was measured,and the tensile elastic modulus of the sheet at the expansion of 100%was obtained. When the rate of “a tensile elastic modulus in the meltflow direction/a tensile elastic modulus in the vertical direction tomelt flows” is closer to 1, the anisotropy is smaller and isotropy ismore excellent.

[Tensile Elastic Modulus of Photosensitive Composition for FlexographicPlate]

The photosensitive composition for a flexographic plate was heated at150° C. and melted, and continuously extruded using a biaxial extruderwith T-dye installed, so as to be molded into a sheet having a thicknessof 2 mm. The detailed conditions for molding the sheet were as follows:

Composition treatment speed: 25 kg/hr;

Unloading speed: 1.0 m/min;

Extruder temperature: Input port 140° C., T-dye adjusted to 160° C.;

Screw: Full-flight;

Extruder L/D: 20; and

T-dye: Width 200 mm, lip 2.5 mm.

The obtained sheet was exposed to light by irradiating active light for10 minutes using an exposure machine (Type JE-A3-SS from Nihon DenshiSeiki Co., Ltd.) with an ultraviolet fluorescent lamp of 20 W installed,and dried for 30 minutes with a warm air dryer at 60° C. Two pieces ofthis exposed sheet were used, and the tensile elastic modulus of the onewas measured along with the melt flow direction during molding, and thetensile elastic modulus of the other was measured along with thevertical direction to the melt flows during molding. The measurementprocedures were as follows. The sheet was expanded to 100% with thetension rate of 300 mm/min using TENSILON universal testing instrumentRTC-1210 from ORIENTEC Co., LTD., and the tension stress at theexpansion of 100% during the process was measured, and the tensileelastic modulus of the sheet at the expansion of 100% was obtained. Whenthe rate of “a tensile elastic modulus in the melt flow direction/atensile elastic modulus in the vertical direction to melt flows” iscloser to 1, the anisotropy is smaller and isotropy is more excellent.

[Tension Set of Photosensitive Composition for Flexographic Plate]

With the same method as that for the measurement of the tensile elasticmodulus, the photosensitive composition for a flexographic plate wasexposed to light and a sheet thereof was obtained. The tension set ofthis sheet was measured using the TENSILON universal testing instrumentin accordance with ASTM 412. In specific, a sample shape used was Die A,and the sheet was expanded to the expanding rate of 200% with theinter-marker distance before expansion being 40 mm, the expanded statewas kept for 10 minutes, and then rapidly contracted without rebounding,leave as it was for 10 minutes, and thereafter the inter-marker distancewas measured and thereby the tension set was obtained from the belowformula. When the value of the tension set is lower, the rubberelasticity is more excellent.

Tension set (%)=(L1−L0)/L0*100

-   -   L0: inter-marker distance before expansion (mm)    -   L1: inter-marker distance after contracting and leaving for 10        minutes (mm)        Incidentally, in this measurement, two pieces of the sheet were        used, and the value of the one along with the melt flow        direction during molding was measured, and the value of the        other along with the vertical direction to the melt flows during        molding was measured, and each of the value was recorded.

[Abrasion Resistance of Photosensitive Composition for FlexographicPlate]

With the same method as that for the measurement of the tensile elasticmodulus, the photosensitive composition for a flexographic plate wasexposed to light and a sheet thereof was obtained. Next, the obtainedsheet and a water-resistant paper No. 1000 were rubbed with areciprocating motion and conditions of the load of 100 g and the speedof 6000 mm/sec using HEiDON abrasion tester (from Shinto Scientific Co.,Ltd.), and the abrasion amount of the surface of both sheets after 1000cycles were measured. Incidentally, this parameter is indicated as anindex when Comparative Example 1 is regarded as 100. When the index islarger, the abrasion resistance is more excellent.

[Ink-Swelling Resistance of Photosensitive Composition for FlexographicPlate]

With the same method as that for the measurement of the tensile elasticmodulus, the photosensitive composition for a flexographic plate wasexposed to light and a sheet thereof was obtained. Next, a sheet ofwhich weight was measured in advance was put into isopropyl alcohol.After 60 minutes, the sheet was taken out, and excess isopropyl alcoholwas wiped out to measure the weight of the sheet. With the weight rate“the weight of sheet after test/the weight of sheet before test”, theink-swelling resistance was measured. In this parameter, when the rateis closer to 100%, the ink-swelling resistance is more excellent.

[Printing Properties (Gel Content)]

The gel content is a scale to see the level of cross-linking after thephotosensitive composition for a flexographic plate is cured.

First, with the same method as that for the measurement of the tensileelastic modulus, the photosensitive composition for a flexographic platewas exposed to light and a sheet thereof was obtained. Next, a sheet ofwhich weight was measured in advance was soaked in toluene overnight,and then taken out. Next, the sheet taken out from toluene was dried at70° C. under reduced pressure until the weight loss could not be seenany more. Then, the weight of the sheet after drying was measured. Thegel content was obtained from the following equation:

Gel content (%)=(w1/w0)*100.

In the equation, w0 is the weight of the sheet before soaking intoluene, and w1 is the weight of the sheet after soaking in toluene anddrying.

Production Example 1

In a pressure-resistant reactor, 23.3 kg of cyclohexane, 66.1 millimolesof N,N,N′,N′-tetramethylethylenediamine (hereinafter, referred to asTMEDA), and 1.19 kg of styrene were introduced, and while the contentwas stirred at 40° C., 143.6 millimoles of n-butyllithium was addedthereto. Polymerization was carried out for one hour while thetemperature was increased to 50° C. The polymerization conversion ratioof styrene was 100%. Subsequently, 7.00 kg of isoprene was continuouslyadded to the reactor over one hour while the temperature was controlledto be maintained at 50° C. to 60° C. After the addition of isoprene wascompleted, polymerization was carried out for another one hour. Thepolymerization conversion ratio of isoprene was 100%. Next, 31.1millimoles of tetramethoxysilane was added thereto as a coupling agent,the coupling reaction was carried out for two hours, and thus, astyrene-isoprene block copolymer that served as a block copolymer B, wasformed. Next, 28.7 millimoles of methanol was added to the reactor, anda portion of the active terminals of the styrene-isoprene blockcopolymer was deactivated. Thus, a styrene-isoprene block copolymer thatserved as a block copolymer C, was formed. Thereafter, 1.81 kg ofstyrene was continuously added thereto over one hour while thetemperature was controlled to be maintained at 50° C. to 60° C. Afterthe addition of styrene was completed, polymerization was carried outfor another one hour, and thus a styrene-isoprene-styrene blockcopolymer that served as a block copolymer A was formed. Thepolymerization conversion ratio of styrene was 100%. Thereafter, 287.2millimoles of methanol as a polymerization terminator was added theretoand mixed thoroughly therein, and thus the reaction was terminated.Incidentally, the amounts of the various reagents used in the reactionare summarized in Table 1.

A portion of the reaction liquid thus obtained was taken, and the weightaverage molecular weights of the various block copolymers and the weightaverage molecular weight of the block copolymer composition, the weightaverage molecular weights of the various styrene polymer blocks, theweight average molecular weights of the various isoprene polymer blocks,the styrene unit contents of the various block copolymers, the styreneunit content of the block copolymer composition, the vinyl bond contentof the isoprene polymer block, and the mass ratios of the various blockcopolymers were determined. These values are presented in Table 2.

To 100 parts of the reaction liquid obtained as described above(including 30 parts of polymer components), 0.3 part of2,6-di-t-butyl-p-cresol was added and mixed as an antioxidant, and themixed solution was added dropwise in small amounts to hot water that hadbeen heated to 85° C. to 95° C. to thereby volatilize the solvent, andthus a separating material was obtained. This separating material waspulverized and dried in hot air at 85° C., and thus a block copolymercomposition of Production Example 1 was collected. The type A hardnessand the anisotropic index of the block copolymer composition ofProduction Example 1 were measured. These values are presented in Table2.

TABLE 1 Prodution Examples 1 2 3 4 5 6 7 cyclohexane (kg) 23.3 23.3 23.323.3 23.3 23.3 23.3 TMEDA (millimoles) 66.1 120.0 71.2 82.7 2.5 2.2 1.9n-butyllithium (millimoles) 143.6 166.7 154.8 159.1 164.7 147.7 129.3Styrene (kg) 1.19 1.33 1.30 1.32 1.55 1.49 1.16 [Polymerization firststage] n-butyllithium (millimoles) — — — — — — — [Polymerization firststage • Re-addition] styrene (kg) — — — — — — — [Polymerization firststage • Re-addition] isoprene (kg) 7.00 7.00 5.20 6.00 5.20 5.60 5.30[Polymerization second stage] butadiene (kg) — — — — — — —[Polymerization second stage] dimethylmethoxysilane (millimoles) — — — —60.1 — — [After polymerization second stage] tetramethoxysilane(millimoles) 31.1 36.1 37.7 34.5 — 28.7 16.2 [After polymerizationsecond stage] methanol (millimoles) 28.7 25.0 23.2 23.9 23.1 31.0 18.1[After polymerization second stage] styrene (kg) 1.81 1.67 3.50 2.683.25 2.91 3.54 [Polymerization third stage] methanol (millimoles) 287.2333.4 309.6 318.2 329.4 295.4 258.6 [After polymerization third stage]Prodution Examples 8 9 10 11 12 13 14 cyclohexane (kg) 23.3 23.3 23.323.3 23.3 23.3 23.3 TMEDA (millimoles) 0.8 2.6 1.3 1.9 2.3 1.2 1.82n-butyllithium (millimoles) 14.1 172.4 90.0 128.8 156.0 77.9 121.5Styrene (kg) 1.24 1.90 0.90 1.50 2.20 4.15 1.25 [Polymerization firststage] n-butyllithium (millimoles) 43.2 — — — — — — [Polymerizationfirst stage • Re-addition] styrene (kg) 0.85 — — — — — — [Polymerizationfirst stage • Re-addition] isoprene (kg) 7.05 8.10 8.20 7.00 5.60 5.20 —[Polymerization second stage] butadiene (kg) — — — — — — 7.50[Polymerization second stage] dimethylmethoxysilane (millimoles) — — — —— — — [After polymerization second stage] tetramethoxysilane(millimoles) — 34.5 — — — — — [After polymerization second stage]methanol (millimoles) — — — — — — — [After polymerization second stage]styrene (kg) 0.86 — 0.90 1.50 2.20 0.65 1.25 [Polymerization thirdstage] methanol (millimoles) 114.6 344.8 180.0 257.6 312.0 155.9 243.0[After polymerization third stage]

TABLE 2 Production Examples 1 2 3 4 5 6 7 8 Block copolymer A Relativelysmall weight average molecular weight of 9000 9000 9100 9000 10000 90009000 12000 styrene block Ar1 [Mw(Ar1a)] Relatiely large weight averagemolecular weight of 34000 26000 94000 56000 153000 123000 57000 100000styrene block Ar2 [Mw(Ar2a)] Mw(Ar2a)/Mw(Ar1a) 3.8 2.9 10.3 6.2 15.313.7 6.3 8.3 Weight average molecular weight of 72000 68000 54000 5770070000 62000 59000 123000 isoprene block Da Vinyl bond content ofisoprene block Da (% by mol) 22 60 25 30 7 7 7 9 Weight averagemolecular 115000 103000 157100 122700 215000 198000 124000 235000 weightof block copolymer A Styrene unit content of block copolymer A (%) 65 6486 76 76 54 59 53 Block copolymer B Weight average molecular 9000 90009100 9000 10000 9000 9000 12000 weight of styrene block Arb Constituentof conjugated diene block copolymer B isoprene isoprene isopreneisoprene isoprene isoprene isoprene isoprene Weight average molecularweight 72000 68000 54000 57700 70000 62000 59000 123000 of conjugateddiene block Db Vinyl bond content 22 60 25 30 7 7 7 9 of conjugateddiene block Db (% bymol) Weight average molecular weight 226800 215600176400 186800 117000 284000 276000 147000 of block copolymer B Styreneunit content of block copolymer B (%) 14 16 20 18 17 21 18 19 Blockcopolymer A/ 48/52 51/48 49/51 43/57 52/48 80/20 78/22 34/66 Blockcopolymer B (mass ratio) Block copolymer C Weight average molecularweight 9000 9000 9100 9000 10000 9000 9000 — of styrene block Arc Weightaverage molecular weight 72000 68000 54000 57700 71000 62000 59000 — ofisoprene block Dc Rate relative to block copolymer 11 10 10 9 10 11 10 0composition overall (%) Polymer component presumed to be block copolymerD Weight average molecular weight 295000 — 254000 — 230000 390000 320000— Styrene unit content (%) 27 — 37 — 55 43 38 — Rate relative to blockcopolymer 3 — 4 — 10 9 9 0 composition overall (%) Block copolymercomposition (overall) Weight average molecular weight 168000 150000142000 136000 149000 215000 128000 175000 Styrene unit content (%) 30 3048 40 48 44 47 30 Type A hardness 43 41 64 55 65 61 68 62 Anisotropicindex 1.1 1.2 1.1 1.3 1.1 1.2 3.5 1.3 Production Examples 9 10 11 12 1314 Block copolymer A Relatively small weight average molecular weight of— — — — 15000 — styrene block Ar1 [Mw(Ar1a)] Relatiely large weightaverage molecular weight of — — — — 76000 — styrene block Ar2 [Mw(Ar2a)]Mw(Ar2a)/Mw(Ar1a) — — — — 5.1 — Weight average molecular weight of — — —— 133000 — isoprene block Da Vinyl bond content of isoprene block Da (%by mol) — — — — 7 — Weight average molecular — — — — 224000 — weight ofblock copolymer A Styrene unit content of block copolymer A (%) — — — —48 — Block copolymer B Weight average molecular 11000 10000 13000 15000— 11000 weight of styrene block Arb Constituent of conjugated dieneblock copolymer B isoprene isoprene isoprene isoprene — butadiene Weightaverage molecular weight 70000 50000 45000 30000 — 50000 of conjugateddiene block Db Vinyl bond content 7 7 7 7 — 11 of conjugated diene blockDb (% bymol) Weight average molecular weight 270000 120000 116000 90000— 125000 of block copolymer B Styrene unit content of block copolymer B(%) 19 18 30 44 — 25 Block copolymer A/ — — — — — — Block copolymer B(mass ratio) Block copolymer C Weight average molecular weight 11000 — —— — — of styrene block Arc Weight average molecular weight 71000 — — — —— of isoprene block Dc Rate relative to block copolymer 28 0 0 0 0 0composition overall (%) Polymer component presumed to be block copolymerD Weight average molecular weight — — — — — — Styrene unit content (%) —— — — — — Rate relative to block copolymer 0 0 0 0 0 0 compositionoverall (%) Block copolymer composition (overall) Weight averagemolecular weight 214000 120000 116000 90000 224000 125000 Styrene unitcontent (%) 19 18 30 44 48 25 Type A hardness 44 46 53 92 98 62Anisotropic index 2.5 2.2 6.5 1.8 1.5 7.5

Production Examples 2 to 4

Compositions of Production Examples 2 to 4 were collected in the samemanner as in Production Example 1, except that the amounts of TMEDA,styrene, n-butyllithium, isoprene, tetramethoxysilane, and methanol werechanged respectively as indicated in Table 1. The same measurements asin Production Example 1 were carried out for the block copolymercompositions for Production Examples 2 to 4. The results are presentedin Table 2.

Production Example 5

The block copolymer composition of Production Example 5 was collected inthe same manner as in Production Example 1, except that the 31.1millimoles of tetramethoxysilane was replaced with 60.1 millimoles ofdimethyldimethoxysilane, and the amounts of TMEDA, styrene,n-butyllithium, isoprene, and methanol were changed respectively asindicated in Table 1. The same measurements as in Production Example 1were carried out for the block copolymer composition for ProductionExample 5. The results are presented in Table 2.

Production Examples 6 to 7

Compositions of Production Examples 6 to 7 were collected in the samemanner as in Production Example 1, except that the amounts of TMEDA,styrene, n-butyllithium, isoprene, tetramethoxysilane, and methanol werechanged respectively as indicated in Table 1. The same measurements asin Production Example 1 were carried out for the block copolymercompositions for Production Examples 6 to 7. The results are presentedin Table 2.

Production Example 8

In a pressure-resistant reactor, 23.3 kg of cyclohexane, 0.8 millimolesof TMEDA, and 1.24 kg of styrene were introduced, and while the contentwas stirred at 40° C., 14. 1 millimoles of n-butyllithium was addedthereto. Polymerization was carried out for one hour while thetemperature was increased to 50° C. Next, 43.2 millimoles ofn-butyllithium was added, sequentially 0.85 kg of styrene wascontinuously added over 30 minutes, and the polymerization was continuedfor one hour. The polymerization conversion ratio of styrene was 100%.Subsequently, 7.05 kg of isoprene was continuously added to the reactorover one hour while the temperature was controlled to be maintained at50° C. to 60° C. After the addition of isoprene was completed,polymerization was carried out for another one hour. The polymerizationconversion ratio of isoprene was 100%. Next, 0.86 kg of styrene wascontinuously added for 30 minutes, and after the addition of styrene wascompleted, polymerization was carried out for another one hour, and thustwo kinds of styrene-isoprene-styrene block copolymer that served as ablock copolymer A and a block copolymer B were formed. Thepolymerization conversion ratio of styrene was 100%. Thereafter, 114. 6millimoles of methanol as a polymerization terminator was added theretoand mixed thoroughly therein, and thus the reaction was terminated. Aportion of the reaction liquid thus obtained was taken, and the samemeasurements as in Production Example 1 were carried out. These valuesare presented in Table 2. Further operations were carried out in thesame manner as in Production Example 1, and thus a block copolymercomposition of Production Example 8 was collected.

Production Example 9

In a pressure-resistant reactor, 23.3 kg of cyclohexane, 2.6 millimolesof TMEDA, and 1.90 kg of styrene were introduced, and while the contentwas stirred at 40° C., 172.4 millimoles of n-butyllithium was addedthereto. Polymerization was carried out for one hour while thetemperature was increased to 50° C. The polymerization conversion ratioof styrene was 100%. Subsequently, 8.10 kg of isoprene was continuouslyadded to the reactor over one hour while the temperature was controlledto be maintained at 50° C. to 60° C. After the addition of isoprene wascompleted, polymerization was carried out for another one hour. Thepolymerization conversion ratio of isoprene was 100%. Next, 34.5millimoles of tetramethoxysilane was added thereto as a coupling agent,the coupling reaction was carried out for two hours, and thus, abranched styrene-isoprene-styrene block copolymer that served as a blockcopolymer B was formed. Thereafter, 344. 8 millimoles of methanol as apolymerization terminator was added to a solution in which thestyrene-isoprene block copolymer including an active terminal remained,and mixed thoroughly; thus the reaction was terminated. A portion of thereaction liquid thus obtained was taken, and the same measurements as inProduction Example 1 were carried out. These values are presented inTable 2. Further operations were carried out in the same manner as inProduction Example 1, and thus a block copolymer composition ofProduction Example 9 was collected.

Production Example 10

In a pressure-resistant reactor, 23.3 kg of cyclohexane, 1.3 millimolesof TMEDA, and 0.90 kg of styrene were introduced, and while the contentwas stirred at 40° C., 90.0 millimoles of n-butyllithium was addedthereto. Polymerization was carried out for one hour while thetemperature was increased to 50° C. The polymerization conversion ratioof styrene was 100%. Subsequently, 8.20 kg of isoprene was continuouslyadded to the reactor over one hour while the temperature was controlledto be maintained at 50° C. to 60° C. After the addition of isoprene wascompleted, polymerization was carried out for another one hour. Thepolymerization conversion ratio of isoprene was 100%. Thereafter, 0.90kg of styrene was continuously added thereto over one hour while thetemperature was controlled to be maintained at 50° C. to 60° C. Afterthe addition of styrene was completed, polymerization was carried outfor another one hour, and thus a styrene-isoprene-styrene blockcopolymer was formed. The polymerization conversion ratio of styrene was100%. Thereafter, 180.0 millimoles of methanol as a polymerizationterminator was added thereto and mixed thoroughly therein, and thus thereaction was terminated. A portion of the reaction liquid thus obtainedwas taken, and the same measurements as in Production Example 1 werecarried out. These values are presented in Table 2. Further operationswere carried out in the same manner as in Production Example 1, and thusa block copolymer composition of Production Example 10 was collected.

Production Examples 11 to 13

Block copolymer compositions of Production Examples 11 to 13 werecollected in the same manner as in Production Example 10, except thatthe amounts of TMEDA, styrene, n-butyllithium, isoprene and methanolwere changed respectively as indicated in Table 1. For the blockcopolymer compositions of Production Examples 11 to 13, the samemeasurements as in Production Example 1 were carried out. The resultsare presented in Table 2.

Production Example 14

The block copolymer composition of Production Examples 14 was collectedin the same manner as in Production Example 10, except that the 8.20 kgof isoprene was replaced with 7.50 kg of butadiene, and the amounts ofTMEDA, styrene, n-butyllithium, and methanol were changed respectivelyas indicated in Table 1. For the block copolymer composition ofProduction Example 14, the same measurements as in Production Example 1were carried out. The results are presented in Table 2.

Example 1

Using a kneader, 100 parts of the block copolymer composition ofProduction Example 1, 10 parts of liquid polybutadiene (NISSO-PB-B-1000from NIPPON SODA CO., LTD.), and 2 parts of 2,6-di-t-butyl-p-cresol werekneaded at 170° C. Sequentially, the kneading temperature was dropped to130° C., and 5 parts of 1,4-butadioldiactylate, 5 parts of1,6-hexandioldimethacrylate, 0.01 parts of methylhydroquinone, and 0.8parts of benzoin iso-propyl ether were added thereto and kneaded toobtain a photosensitive composition for a flexographic plate ofExample 1. For this photosensitive composition for a flexographic plate,a tensile elastic modulus, a tension set, abrasion resistance,ink-swelling resistance, and a gel content were measured. The resultsare represented in Table 3.

TABLE 3 Example Example Example Example Comp. Comp. Comp. 1 2 3 4 Ex. 1Ex. 2 Ex. 3 Blend (parts) Block copolymer Production Example 1 100 — — —— — — composition Production Example 2 — 100 — — — — — ProductionExample 3 — — 100 — — — — Production Example 4 — — — 100 — — —Production Example 5 — — — — 100 — — Production Example 6 — — — — — 100— Production Example 7 — — — — — — 100 Production Example 8 — — — — — —— Production Example 9 — — — — — — — Production Example 10 — — — — — — —Production Example 11 — — — — — — — Production Example 12 — — — — — — —Production Example 13 — — — — — — — Production Example 14 — — — — — — —Weight average molecular weight overall 168000 150000 142000 136000149000 255000 128000 Styrene unit content overall (%) 30 30 48 40 48 4447 Type A hardness 43 41 64 55 65 61 68 Properties 100% tensile elasticmodulus 0.82 1.10 1.68 1.37 1.44 1.25 1.22 in vertical direction to meltflows (Mpa) Tension set in vertical direction to melt flows (%) 3 4 4 44 4 5 100% tensile elastic modulus 0.93 1.35 1.88 1.67 1.71 1.40 1.55 inmelt flow direction (Mpa) Tension set in melt flow direction (%) 4 6 6 55 5 7 Rate of tensile elastic modulus 1.1 1.2 1.1 1.2 1.2 1.1 1.3 meltflow direction/vertical direction to melt flows Abrasion resistance(index) 80 95 160 140 150 165 155 Printing properties (gel content) (%)92 99 91 98 67 70 65 Ink-swelling resistance (%) 102 101 101 101 101 101101 Comp. Comp. Comp. Comp. Comp. Comp. Comp. Ex. 4 Ex. 5 Ex. 6 Ex. 7Ex. 8 Ex. 9 Ex. 10 Blend (parts) Block copolymer Production Example 1 —— — — — — — composition Production Example 2 — — — — — — — ProductionExample 3 — — — — — — — Production Example 4 — — — — — — — ProductionExample 5 — — — — — — — Production Example 6 — — — — — — — ProductionExample 7 — — — — — — — Production Example 8 100 — — — — — — ProductionExample 9 — 50 — — — — — Production Example 10 — — 50 — — — — ProductionExample 11 — — — 100 — — — Production Example 12 — — — — 100 — —Production Example 13 — 50 50 — — 100 — Production Example 14 — — — — —— 100 Weight average molecular weight overall 175000 220000 172000116000 90000 224000 125000 Styrene unit content overall (%) 30 34 33 3044 48 25 Type A hardness 62 55 53 53 92 98 62 Properties 100% tensileelastic modulus 0.61 0.71 0.78 0.83 3.79 5.27 0.63 in vertical directionto melt flows (Mpa) Tension set in vertical direction to melt flows (%)5 4 2 5 14 60 8 100% tensile elastic modulus 1.10 0.75 0.89 3.71 4.956.53 2.30 in melt flow direction (Mpa) Tension set in melt flowdirection (%) 9 6 5 4.5 15 65 12 Rate of tensile elastic modulus 1.8 1.11.1 1.2 1.3 12 3.7 melt flow direction/vertical direction to melt flowsAbrasion resistance (index) 90 105 100 85 150 170 90 Printing properties(gel content) (%) 72 71 75 70 43 35 90 Ink-swelling resistance (%) 103106 106 107 101 101 110

Examples 2 to 4

Photosensitive compositions for a flexographic plate of Examples 2 to 4were obtained in the same manner as in Example 1, except that the blockcopolymer composition to be used was changed to the block copolymercompositions of Production Examples 2 to 4. For these photosensitivecomposition for a flexographic plate of Examples 2 to 4, the samemeasurements as in Example 1 were carried out. The results are presentedin Table 3.

Comparative Examples 1 to 10

Photosensitive compositions for a flexographic plate of ComparativeExamples 1 to 10 were obtained in the same manner as in Example 1,except that the block copolymer composition to be used was changed tothe block copolymer compositions of Production Examples 5 to 14. Forthese photosensitive composition for a flexographic plate of ComparativeExamples 1 to 10, the same measurements as in Example 1 were carriedout. The results are presented in Table 3.

Followings can be seen from Table 2 and Table 3. That is, the blockcopolymer composition for a flexographic plate (Production Examples 1 to4) of the present invention had excellent flexibility and crosslinkingproperties, its anisotropy was small, and the photosensitive compositionfor a flexographic plate of the present invention obtained from theblock copolymer composition for a flexographic plate of the presentinvention was provided with both low tension set and high abrasionresistance, which can be said it was excellent in rubber elasticity andabrasion resistance, and was further excellent in ink-swellingresistance and isotropy (Examples 1 to 4). In contrast, the blockcopolymer compositions (Production Examples 5 to 14), which did not fallunder the block copolymer composition for a flexographic plate of thepresent invention, did not have enough of at least one of flexibility,crosslinking properties, and anisotropy, and when these block copolymercompositions were used, the balance of rubber elasticity and abrasionresistance was inferior, and anisotropy was expressed (ComparativeExamples 1 to 10).

1. A block copolymer composition for a flexographic plate, comprising ablock copolymer A represented by the following general formula (A) and ablock copolymer B represented by the following general formula (B),wherein: a content of aromatic vinyl monomer units relative to allmonomer units constituting polymer components of the block copolymercomposition for the flexographic plate is 18% to 70% by mass; a type Ahardness is 25 to 65; and an anisotropic index is 2.0 or less:Ar1^(a)-D^(a)-Ar2^(a)  (A)(Ar^(b)-D^(b))_(n)-x  (B) in the general formula (A) and the generalformula (B), Ar1^(a) and Ar^(b) each represent an aromatic vinyl polymerblock having a weight average molecular weight of 6,000 to 20,000;Ar2^(a) represents an aromatic vinyl polymer block having a weightaverage molecular weight of 25,000 to 400,000, a ratio(Mw(Ar2^(a))/Mw(Ar1^(a))) of the weight average molecular weight ofAr2^(a) and the weight average molecular weight of Ar1^(a) is 2 to 20;D^(a) and D^(b) each represent a conjugated diene polymer block having avinyl bond content of 21% to 70% by mol; X represents a single bond or aresidue of coupling agent; and n represents an integer of 2 or more. 2.The block copolymer composition for a flexographic plate according toclaim 1, wherein the content of aromatic vinyl monomer units relative toall monomer units constituting polymer components of the block copolymercomposition for the flexographic plate is 20% to 70% by mass.
 3. Theblock copolymer composition for a flexographic plate according to claim1, wherein a mass ratio (A/B) of the block copolymer A and the blockcopolymer B is 36/64 to 85/15.
 4. The block copolymer composition for aflexographic plate according to claim 1, further comprising a blockcopolymer C represented by the following general formula (C):Ar^(c)-D^(c)  (C) in the general formula (C), Ar^(c) represents anaromatic vinyl polymer block having a weight average molecular weight of6,000 to 20,000, and D^(c) represents a conjugated diene polymer blockhaving a vinyl bond content of 21% to 70% by mol.
 5. The block copolymercomposition for a flexographic plate according to claim 1, wherein theblock copolymer B is obtained by using a compound including two or moreof at least one kind of functional group selected from an alkoxyl group,an ester group and an epoxy group in one molecule as a coupling agent.6. A photosensitive composition for a flexographic plate comprising theblock copolymer composition for a flexographic plate according to claim1, an ethyleny unsaturated compound having a molecular weight of 5,000or less, and a photopolymerization initiator.
 7. A flexographic platecomprising the photosensitive composition for a flexographic plateaccording to claim
 6. 8. A method for producing the block copolymercomposition for a flexographic plate according to claim 5, the methodcomprising: a first step of forming an aromatic vinyl polymer includingan active terminal by polymerizing an aromatic vinyl monomer using apolymerization initiator in a solvent; a second step of forming anaromatic vinyl-conjugated diene block copolymer including an activeterminal by adding a conjugated diene monomer to a solution obtained inthe first step for polymerization; a third step of forming a blockcopolymer B by adding, as a coupling agent, a compound including two ormore of at least one kind of functional group selected from an alkoxylgroup, an ester group and an epoxy group in one molecule, to a solutionobtained in the second step, in an amount the functional group relativeto the active terminal is less than one equimolar amount; a fourth stepof forming a block copolymer A by adding an aromatic vinyl monomer to asolution obtained in the third step for polymerization; and a fifth stepof collecting the block copolymer composition for the flexographic platefrom a solution obtained in the fourth step; wherein reactions in atleast the second step, the third step, and the fourth step are conductedunder the presence of a Lewis base compound, and an amount of the Lewisbase compound per 1 mol of the active terminal in the polymerizationinitiator is 0.1 to 50 mol.